This one is the start of a long thread with several tweaks listed.
http://www.forumplanet.com/planetdoom/topic.asp?fid=5733&tid=1438663&p=1
This one also has some info. Im looking for the other sites I looked at last night.
http://doom3.filefront.com/file.news?ID=11538
Essentially here is what you want to do.
go into the Doom3/Base folder and open up DoomConfig.cfg in notepad. Be sure to rename and save the original file.
Look for these lines and change them as necessary.
r_gamma "1" 1.2 is the best value, but some systems need up to 1.4
com_videoRam "x" should be set you whatever your video card's ram is ie 64
r_swapinterval "0" default is 0, change it to 1
seta image_useCache "0" change to 1 to help the next tweak work better
seta image_cacheMegs "20" any value from 96 to 256 works. ideally it should be 1/4 your system RAM
seta r_displayrefresh "0" change to whatever your video card's refresh rate is for the resolution you are running ie 72
seta image_cacheMink "200" 20480 and 25600 work well
there are some more tweaks and fps tests, etc but these have worked well on my system. if you want a general idea of what your system will run it at without tweaks bring up the console in the game and type in timedemo demo1
console is alt+ctrl+~