Saw this on Ye Ole Forums. For those of you with ATi cards check out this here post which will hopefully speed up Doom 3's performance on your ATi card by up to 40%. Note that if you have an NVIDIA card this won't work (in fact it will make the game slower), so only try it if you have a Radeon. Also it's important to back up your pk4 files and such first as this will probably break multiplayer. Worth a shot if you want some extra fps.