Quake 3 Arena runs great on my iMac Core Duo 20". I use a custom resolution in Quake 3 so it will run on 1680x1050. Rocket Arena 3 (a mod) runs.
Doom3 (the universal binary came out today) runs well, as expected on a resolution of 1680x1050 the game has moments with only 16 fps.
Changing the resolution to 1280x800 and tweaking (bilinear filtering, no shadows, everything else set to high) the cfg a bit fps never drops below 40 - with nice graphics. I hope Aspyr releases a smp patch for Doom.
I can't wait for Quake 4 for Mac OS, I hope RA4 will run on the Mac.
iMac Core Duo 20", 2 GB RAM, 256 MB VRAM Mac OS X (10.4.4)