Now that Quake 3 is working on morphO 3.6, I've run some benchmarks on my G5. Feel free to add your benchmarks. Using Piru's settings, here's what I have.

Test System:
Powermac g5 @ 2.0Ghz
Radeon 9800 Pro (memory clock 270Mhz, GPU clock 297Mhz)

Using parameters by Piru for benchmarking the driver:
setenv TGLSYNC 0
flushlib tinygl.library

And under Quake 3 (All details maxed out):
r_primitives 2
timedemo 1
demo four

Results:
640x480 - 71.8 FPS
800x600 - 71.1 FPS
1024x768 - 70.2 FPS
1280x1024 - 56.4 FPS

Somewhat strangely, my old Quicksilver 2Ghz with Radeon 8500LE benched much better than this G5. I'm guessing the current build was optimized for the G4? A G5 specific build would be nice.

What are your benchmarks on G5?

Yes that's actually pretty poor numbers, with such low resolution as 1280x1024, only 56fps. Try tweaking the settings with cg_simpleItems "1" .. I remember this helping quite a lot in getting more fps.

I just tried with my 2GHz G5 which has a Radeon X800 currently. I booted from the MorphOS 3.6 CD, used a clean Quake 3 installation and otherwise the same TGLSYNC 0 and r_primitives 2 settings as you. I got 168 FPS at 640x480, 145 FPS at 1280x1024 and 125FPS at 1920x1080.

The 9800 Pro will fare a bit poorer for two reasons. It is slower than the X800 (but not _that_ much slower), and the command buffer can't be DMA driven on that card. However, that should mainly affect FPS at lower resolutions, and you shouldn't be seeing such a sharp decline in FPS at such a low resolution.

I'll try with a Radeon 9800 Pro probably later today at some point, but if I don't get to it before you can do more tests, I'd recommend doing as I did and boot from CD with a fresh Quake 3 installation and seeing how it goes. BTW, to get Quake 3 to start when booting from CD, you have to open up system prefs, go to the networking settings and press 'use' without changing anything.

Ouch, I tried to run a test even on my Ibook G4 1.42ghz.

800x600 - 40 FPS
1024x768 - 20 FPS

It would corrupt and go slower and slower with the audio glitching out and almost come to a hault. Eventually once you go to desktop its has lines/artifacts all over, but once you run or open up windows it fixes itself up and normal again. Other games play well with no graphical errors. I will have to try on my mac mini later to see what numbers it does.

A bit later than I had hoped, but I finally tried the Radeon 9800 Pro in my G5, which should make my config equal to yours. Here are my results:

640x480: 143.5 FPS
1280x1024: 98.4 FPS
1920x1080: 83.9 FPS

However, I noticed something that might explain the difference. You say your card has a 270MHz memory clock and a 297MHz core clock. At least according to the GraphicsBoards utility, my card has a 338MHz memory clock and a 378MHz core clock. My numbers are inline with what to expect from a Radeon 9800 Pro, where as your numbers are below that of even a Radeon 9800 SE.

GPUReview link

Is your card a genuine card or a reflashed one? If it's a reflashed one, it might have been flashed with a firmware that sets it up like some sort of Radeon 9800 SE, which would drastically impair its performance, just like you're experiencing.