A lot of people are surprised at how slowly Doom 3 runs, even with a good computer.
Here is a simple tweak to a text file that will make things much faster and allow you to use a high graphics resolution and detail level:
My games system isn’t the fastest or newest, Athlon XP 2500+, 1gb PC2700 RAM, Radeon 9800 Pro 128mb. With that system Doom 3 defaulted to medium detail and 800×600 resolution (in comparison every other game runs well at 1800×1600 max graphics). I upped the graphics to 1280×1024 and high graphics but got massive slowdowns and jerkiness (from 10 frames/second down to under 1 frame per second) whenever there was a lot of movement on screen.
Then I found this simple trick that made my Doom 3 run faster:
Look for the file C:\Program Files\Doom 3\base\Doomconfig.cfg and open it in a text editor.
Look for the line: seta image_cacheMegs “20″
(mine said 20, yours may have a different number)
This configuration line tells Doom 3 how much of your RAM to use to store image files (such as monsters, walls etc), and when there isn’t room for a file in there it has to be loaded from disk.
I changed it from 20 to 512 (leaving the other half of my RAM for XP and the Doom game to use… if you only have 512mb RAM try changing the number to 128 or 256), and saved the file.
After restarting Doom 3 I was able to run Doom 3 at 1280×1024 ersolution and high graphics with no slowdowns! (averaging 25 frames/second and going as high as 60).
Note: another thing you should do is make sure you have the latest version of DirectX (9.0c) and the drivers for your graphics card (for ATI cards there is a new Beta version of the drivers that makes Doom 3 faster/better).