have u tried lowering the no. of sound channels? (i've mine @ 16) or changing the CacheSizeMegs?? (I have mine increased to 128mb.)

Texture Quality settings make a big difference. My G5 only comes with the 64mb (tiny vram!) ATI 9600. So cos i've only got this low amount of vram for such modern games, like UT2004, i lower the text quality to LOW or NORMAL.
(i keep the main one @ LOW, and the character ones @ NORMAL. The World and physics i have set to HIGH.

It's not really a G5 problem, it's the gfx card... and it's not cos it's a slow gfx card, it's due to the lacking vram. All imho, of course! ;)

However, the sound has a major hit also - simply cos PCs come with sound cards and macs don't.. so turn off any music and also lower the no. of sound channels...

(The UT2004.ini file is inside your application support folder)

I've just been playing the artic level of Fraghouse's Invasion. And i get anywhere between 30/40fps too over 100fps. Depends! ;)
All other mods/levels/gametypes, like Shattered Oasis, FragOps, Atlantis, Jungle Warfare, Red Orchestra. Domain 2049, and a ton more all play VERY smooth and fast. I'm happy :)

HTH :-?