I've got a P4 2.8Ghz, 1GB DDR, Radeon 9800 Pro 128 MB. Doom 3 on high quality @ 1024x768 gave me 35 FPS in an empty corridor, 25-30 if something happened. Shifting down to 800x600 @ medium quality helped tremendously, putting my FPS in the 40 ranges consistently.
Maybe my system isn't tweaked properly, or maybe ATI's OpenGL driver really does suck. I've never had a stability issue, just an in-game performance issue. Anyone have a nice tweak guide to link to?