I'm running a 9500pro, althon xp 2100 @2.0ghz, and 512mb pc2700 ram and the time demo runs at 36fps at 800x600 med quality. During the game, if there are no enemies on the screen it usually runs at 45-60fps (the time demo has lots of enemies and action) and it rarely goes below 20fps in the most intense action sequences.
The game is so dark that playing at a low resolution doesn't hurt image quality much since you can't see the aliased edges that well, and also going from med quality to high quality doesn't look much different either. I'm very satisfied with how my machine runs doom 3 and am happy that my 9500pro will last a while longer.