http://ucguides.savagehelp.com/Doom3/FPSVisuals.htm

I followed the instructions there (i didnt creat an autoexec.cfg though,
i just pasted the tweak into base/Doomconfig.cfg) my performance jumped
from 13 to 40 fps (800*60,High Quality) (!!!)

Sys:XP@2Ghz,256MB DDR@266Mhz,GF4 ti4400@300/650

inredible! Try it!