> 2x DVI/AGP sockets (to support dual DVI/AGP monitors)

AGP or ADC?

> 1. What are the most and least powerful or cheapest
> AGP cards that meet the above requirements?

The most powerful card to meet your requirements is the Radeon X850 XT, followed by the X800 XT (Mac editions of both, of course)*.

> 2. I already own a GIGABYTE GV-R955256D (from a PC)
> which I believe is based on the Radeon 9550. Can I
> use this card in my G5 temporarily with MorphOS [...]?
> Will it work without modification with MorphOS and/or
> can it be flashed to work with OSX?

This R3xx-based card won't work as is, no matter the OS. For a non-Mac card to work with MorphOS and Linux it has to be R5xx-based (AtomBIOS) at least**. Flashing may be worth a try, but now and then 9600 cards have been reported to work badly after flashing.


Edit:
* see comment #10
** While early AtomBIOS versions came already somewhen during R400 series, the MorphOS AtomBIOS parser seems to require the newer AtomBIOS versions that started with the R500 series.

[ Edited by Andreas_Wolf 19.01.2021 - 01:03 ]