These are some of the tweaks I found on the net. I don't have the game so I cannot attest to how well they work myself.
Doom 3 Performance Enhancements
In your DOOM 3 directory, find "DoomConfig.cfg" (x:\Program Files\Doom 3\base). Open DoomConfig.cfg in Notepad.
Find the line:
seta image_cacheMegs "32"
I changed 32 to 96.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
One said: "In the base folder of ur Doom 3 installation, there are several *.pk4 files These files contains all the resources that doom 3 uses, like models, textures, sounds, ? If these files are uncompressed using WinRAR, u will see great speed ups in doom since now, those files no longer need to be extracted".
Be warned that after extraction, your Doom III installation will be over 4GB, but that you can delete the *.pk4 files afterwards.
The second mail came from Bill, and was very detailed and helpful:
"Here are some changes to the DoomConfig.cfg file that some people report to being helpful in performance but not always in FPS btw.
seta image_useCache "1" == originally ? 0
seta image_cacheMegs "256" == Originally 20 (set this to half your total system ram)
seta image_cacheMinK "20480" == originally 200 (change or will crash when u set usecache to 1)
seta com_showFPS "1" == originally 0 to show your FPS
seta com_videoram "128" == Added this value. Memory on video card, default is 64 so this should help
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Also this is an extensive solution:
How to boost your Doom3 performance by 40% on ATI hardware
Also set AF within the game, not in ATI's control panel.