In anticipation of the (hopefully) upcoming MorphOS 2,6 release (introducing "Power Mac G4" support), this thread is intended to summarise the answers given with regard to supported hardware, and hopefully will act as a useful guide and resource for those wishing to purchase a PowerMac.

The onboard devices/ports/expansions differ between models, as does (to a small degree) the support under MorphOS.

The term "Power Mac G4" includes a broad variety of Apple Mac models released between 1999 and 2003.
As per Apple documentation [1], [2] (and Everymac [17] for the Model Identifiers) these are as follows:

PowerMac1,2 "Power Mac G4 (PCI Graphics)"
PowerMac3,1 "Power Mac G4 (AGP Graphics)"
PowerMac3,3 "Power Mac G4 (Gigabit Ethernet)"
PowerMac3,4 "Power Mac G4 (Digital Audio)"
PowerMac3,5 "Power Mac G4 (Quicksilver)" & "Power Mac G4 (Quicksilver 2002)"
PowerMac3,6 "Power Mac G4 (MDD)" & "Power Mac G4 (FW800)" & "Power Mac G4 (MDD 2003)"
PowerMac5,1 "Power Mac G4 (Cube)"

It appears that there never was a PowerMac3,2. Awww.

Apple also produced the X-Serve G4 range, however they're not officially "Power Mac G4" computers, have no dedicated graphics port, and aren't that common by comparison. Also, their model Identifiers are RackMac1.x.

On which models will MorphOS run?

Rumour [16] has it that all "all AGP based Power Mac G4 models" will be supported.
This is supported by developer comments [18], that "there are no plans to support models other than [...] 3,1 - 3,6".

Whilst not conclusive, it does strongly suggest that all PowerMac3,1 - PowerMac3,6 models will be supported. If you need a more definite answer, you'll just have to wait for the release I guess

It seems that there are no plans to support other G4 models [18], which rules out the "Power Mac G4 (PCI Graphics)" and the "Power Mac G4 (Cube)".

This leaves a list of supported models as:

PowerMac3,1 "Power Mac G4 (AGP Graphics)"
PowerMac3,3 "Power Mac G4 (Gigabit Ethernet)"
PowerMac3,4 "Power Mac G4 (Digital Audio)"
PowerMac3,5 "Power Mac G4 (Quicksilver)" & "Power Mac G4 (Quicksilver 2002)"
PowerMac3,6 "Power Mac G4 (MDD)" & "Power Mac G4 (FW800)" & "Power Mac G4 (MDD 2003)"

You will need to ensure that the processor card is also supported, as not all are. (See section on supported processor cards.)

For the sake of brevity, I've not differentiated between models with different processor speeds or graphic cards - the list is just too long, and these things are ultimately upgradeable/interchangeable.
Various other names (Blue-and-white, Sawtooth, Graphite, etc) also seem to be used intermittently, interchangeably, with no clear definition, or just plain wrongly (at least in Ebay listings), so I would be careful about trusting them as an accurate descriptor.

In this summary, I've (eventually) stuck to using the 'Model Identifiers' (PowerMac3,1 - PowerMac3,6).

Grey Areas

According to Everymac [17], the 'Power Mac 3.x' range also included the 'Mac Server G4'.

Whilst Evermac states:

Quote:
...I've not looked any further into this, so it may be something to be aware of when Mac hunting, at least until someone tries it and reports they are definitely working.

Processor boards

All supported models:
--Processor boards with MPC7450 CPUs will be supported*; [19]
--Processor boards with MPC7400/7410 CPUs will be supported*; [20]
--Single processor boards supported under MorphOS;
--Dual processor boards supported under MorphOS; [13]
----N.B. Only 1 processor will be in use on DP models under MorphOS; [12]
--Currently unknown whether 3rd party CPU upgrades will be supported (eg Sonnet, Newer Tech).
--2nd processor may be used at some future time, but not for initial release. [12], [14]

* It is thought that processor boards clocked at lower speeds (<600MHz as a rule of thumb) may use MPC7400/7410 processors, whereas those clocked higher may use MPC7450. If in doubt, and this matters to you, check.
It has been confirmed that processor boards featuring MPC7400/7410 processors will also be supported. [20]

Graphics slot

PowerMac3,1 - PowerMac3,5:
--AGP 2x slot:
----66MHz, 32-bit, 3.3V;
----Supports 1x, 2x cards (and 4x which operate at 2x). [3], [4], [5], [6]

PowerMac3,6:
--AGP 4x slot:
----66MHz, 1.5V cards supported;
----Supports 4x cards only. [7], [8]

All supported models:
--All AGP slots are supported under MorphOS
--Graphics card must be flashed with mac-compatible firmware. [10]

Card expansion slots

PowerMac3,1 - PowerMac3,3:
--3 x PCI slots:
----Will fit 6.88" and 12.283" cards;
----33MHz, 64-bit;
------Supports 32-bit and/or 64-bit cards;
------Supports 3.3V and/or 5V. [3], [4]

PowerMac3,4 - PowerMac3,6:
--4 x PCI slots
----Will fit 6.88" and 12.283" cards;
----33MHz, 64-bit
------Supports 32-bit and/or 64-bit cards;
------Supports 3.3V and/or 5V. [5], [6], [7], [8]

All supported models:
--All PCI slots are supported under MorphOS, although:
----Any card with its own BIOS (eg SCSI Raid) must be Mac-compatible in order to be available at boot time; [10], [11]
----Cards with BIOS which is not Mac-compatible (eg SCSI?), etc should be useable under MorphOS, however will not be available until after MorphOS has loaded. [11]

Memory banks

PowerMac3,1:
--4 x DIMM banks;
--Supports PC100 SDRAM (Max 1.5GB);
----3.3V, 168-pin, non-ECC, non-parity, unbuffered, non-registered;
----Maximum of 16 devices (ie may be double-sided, but no more than 16 chips in total) per DIMM. [3]

PowerMac3,3:
--4 x DIMM banks;
--Supports PC100 SDRAM (Max 1.5GB);
----3.3V, 168-pin, unbuffered, non-registered (does not mention non-ECC or non-parity);
----Maximum of 16 devices (ie may be double-sided, but no more than 16 chips in total) per DIMM. [4]

PowerMac3,4 - PowerMac3,5:
--3 x DIMM banks;
--Supports PC133 SDRAM (Max 1.5GB);
----3.3V, 168-pin, unbuffered, non-registered (does not mention non-ECC or non-parity);
----Maximum of 16 devices (ie may be double-sided, but no more than 16 chips in total) per DIMM. [5], [6]

PowerMac3,6:
--4 x DIMM banks;
--Supports PC2100 for 133MHz bus models, PC2700 for 167MHz bus models (Max 2GB);
----2.5V, 184-pin DDR, non-ECC, non-parity, unbuffered, non-registered;
----Maximum of 16 devices (ie may be double-sided, but no more than 16 chips in total) per DIMM. [7], [8]

All supported models:
--All memory banks supported under MorphOS;
--Maximum of 1.5GB available to MorphOS; [12]
--All memory shared between both processors for DP models. [8], [12]
--Anecdotally, PowerMac3,1 - PowerMac3,3 may also support 2GB ram. [15]
--Anecdotally, all models require memory to be "low density". [15]
(If you can find an agreed, useful definition for "low density", please post a link)

Onboard PATA

PowerMac3,1 - PowerMac3,5:
--1 x ATA3 (EIDE) port;
--1 x ATA5 (ATA66) port; [12]

PowerMac3,6:
--1 x ATA3 (EIDE) port;
--1 x ATA5 (ATA66) port;
--1 x ATA6 (ATA100) port; [12]

All models:
--All ATA ports supported under MorphOS. [12]

Onboard Firewire

PowerMac3,1 - PowerMac3,6:
--2 x FW400 ports; [3], [4], [5], [6], [7], [8]

"Power Mac G4 (FW800)" only:
--1 x FW800 ports (in addition to FW400 ports); [8]

All supported models:
--All FW ports supported under MorphOS; [10]
--Usefulness subject to Helois firewire stack development. [10]

Onboard USB

PowerMac3,1 - PowerMac3,5:
--2 x USB 1.1 ports (rear); [3], [4], [5], [6]

PowerMac3,6:
--2 x USB 1.1 ports (rear); [7], [8]
--Rumoured to work as USB 2.0, as based on NEC uPD720101 chip (at least on on some models). [9]
----Possible 'unofficial' onboard USB 2.0 functionality not yet supported under MorphOS;

All supported models:
--All USB 1.1 Ports Supported. [13]

Onboard Audio

PowerMac3,1 - PowerMac3,3:
--1 x Audio In;
--1 x Audio Out. [3], [4]

PowerMac3,4 - PowerMac3,5:
--1 x Audio Out;
--1 x Speaker Mini-jack (Apple proprietary audio-out port); [5], [6]

PowerMac3,6:
--1 x Audio In;
--1 x Audio Out;
--1 x Speaker Mini-jack (Apple proprietary audio-out port); [7], [8]

Summary:
--PowerMac3,1 - PowerMac3,3:
----Audio support on unlikely for first release; [12]
--PowerMac3,4 - PowerMac3,6:
----Audio Out supported;
----Audio In not supported in first release;
----Software volume control unlikely in first release;
----Support for Apple Mini-Jack unknown (and who really cares?). [12]

Onboard LAN

PowerMac3,1:
--1 x 10/100 LAN port. [3]

PowerMac3,3 - PowerMac3,6:
--1 x 10/100/1000 port. [4], [5], [6], [7], [8]

All supported models
--LAN Port Supported under MorphOS. [12]

Onboard Airport (/Extreme) slot

PowerMac3,1 - PowerMac3,6 (excluding "Power Mac G4 (FW800)"):
--1 x Airport connector. [3], [4], [5], [6], [7]

"Power Mac G4 (FW800)"
--1 x Airport Extreme connector; [8]

All supported models
--Airport (/Extreme) slot not supported under MorphOS;
--No specific plans in place for future support. [10]
--(imho: appears to simply be another port on the PCI bus, with no bridge, so may 'just work'). [8] (Fig 2.1)

Internal Bluetooth connector

PowerMac3,1 - PowerMac3,6 (excluding "Power Mac G4 (FW800)"):
--Not present [1], [2]

"Power Mac G4 (FW800)"
--Internal connector; [2]

All supported models
--No Bluetooth stack available for MorphOS, so chip support is (almost) irrelevant; (No reference)
--(imho appears to be mounted via USB, so BT HID devices may 'just work' as with the Mac-mini (If BT HID devices are supported for accessing firmware)). [8] (Fig 2.1)

Modem

All supported models:
--Modem not supported; [10]
--No specific plans in place for future support. [10]
--(Modem differs between models). [1], [2]

Other useful information:

MDD/FW800:
-----Guide to reclocking 133MHz bus to 167MHz bus
-----Guide to overclocking Dual processor board (also includes info on voltage regulator jumpers)

Older models:
-----Guide to overclocking Power Mac Single/Dual processor (100MHz bus) models
-----Guide to overclocking Power Mac (Quicksilver) Single processor models

General:
-----Guide (and software) for reflashing Radeon cards for PPC Mac use

Useful comparison sites (Thanks to g4QS_redux):
apple-history
Everymac
Mac HW database tool(Windows/MacOS)

Thanks to all who contributed,



Rich

PS Apologies for any inaccuracies, incompleteness, etc.
PPS Any complaints can be addressed to the usual place.


References:
[1] - Apple online documentation "Power Mac G4: How to differentiate between models, Part 1", accessed 25/8/10 from http://support.apple.com/kb/ht3082
[2] - Apple online documentation "Power Mac G4: How to differentiate between models, Part 1", accessed 25/8/10 from http://support.apple.com/kb/TA25585?viewlocale=en_US
[3] - Apple Documentation "Setting up your Power Mac G4 (PCI/AGP)", accessed 25/8/10 from here
[4] - Apple Documentation "Setting up your Power Mac G4 (Gigabit Ethernet)", accessed 25/8/10 from here
[5] - Apple Documentation "Setting up your Power Mac G4 (Digital Audio)", accessed 25/8/10 from here
[6] - Apple Documentation "Setting up your Power Mac G4 (Quicksilver)", accessed 25/8/10 from here
[7] - Apple Documentation "Setting up your Power Mac G4 (MDD)", accessed 25/8/10 from here
[8] - Apple Documentation "Power Mac G4 (Legacy)", accessed 25/8/10 from here
[9] - Review Site, This page
[10] - Morphzone forum, this thread, [https://morph.zone/modules/newbb_plus/viewtopic.php?forum=11&topic_id=7332&sortname=&sortorder=&sortdays=&viewmode=flat&order=0&start=1]here[/url], Jacadcaps (MorphOS developer)
[11] - Morphzone forum, this thread, [https://morph.zone/modules/newbb_plus/viewtopic.php?forum=11&topic_id=7332&sortname=&sortorder=&sortdays=&viewmode=flat&order=0&start=12]here[/url], Golem (Morphzone moderator)
[12] - Morphzone forum, this thread, [https://morph.zone/modules/newbb_plus/viewtopic.php?forum=11&topic_id=7332&sortname=&sortorder=&sortdays=&viewmode=flat&order=0&start=18]here[/url], Pega-1 (MorphOS developer)
[13] - Morphzone forum, Thread, post #1, MorphOS_Team (MorphOS developer(s))
[14] - Morphzone forum, Thread, page #4-5, Krashan (MorphOS developer)
[15] - Morphzone forum, this thread, [https://morph.zone/modules/newbb_plus/viewtopic.php?forum=11&topic_id=7332&sortname=&sortorder=&sortdays=&viewmode=flat&order=0&start=29]here[/url], Jim
[16] - Morphzone forum, Thread, post #223, Andreas_Wolf (rumour-monger )
[17] - Mac information list, Everymac website, accessed 26/8/10 from here
[18] - Morphzone forum, this thread, here, Jacadcaps (MorphOS Developer)
[19] - Morphzone forum, Thread, Post #4, Pega-1 (MorphOS Developer)
[20] - Morphzone forum, this thread, here, Pega-1 (MorphOS Developer)

Plus links to external sites.

["..hoping that each edit would be the edit home."]

[Latest edits: error correction re max memory on PowerMac3,1; removed duplicated (incorrect) links; updated comments on supported processor cards; changed ref [19] to point to more relevant post, added further information wrt processor cards]