Enigmatic_Cat's DOOM III OPTIMIZATION guide for ATI cards (and possibly NVidia)...

[vizionblind]

from an IGN forum user

NOTE FOR NVIDIA USERS
I've used the program Rage3d tweak to edit my OpenGL settings (which is ATI only). You can probably use an equivalent program for NVidia cards (Rivatuner at Guru3d. I can't help you find the Rivatuner tweaks though, as I don't use that program, so you'll have to find them yourself.

My Specs:
Athlon XP 2500-M @ 2.53Ghz
1GB DDR
9700 Pro @ 380/360

My Settings:
Resolution: 1024x768
Detail: High

Here are a few optimizations I found that greatly increases my fps speed, and keep them more consistent.

For users of High Detail Mode - First thing, if you're currently running the game in High detail mode, you'll want to go into the Doom III cfg file which is located in your Doom3/base folder. It's called "DoomConfig.cfg". Second, look for the line that says "seta image_anisotropy '8'" (you'll remember that High Detail automatically sets this to 8), and change it to a lower value. You probably won't notice the difference in IQ, and the increase in framerate will help alot. I set mine to 4, though you can set it to whatever you feel comfortable with. Remember, every time you switch your detail settings to medium/low, and back to high, it will set Anisotropy to 8 in the cfg file, so you'll have to change it again.

For users of Low Detail Mode - Don't worry about anything, the aformentioned optimizations were Anisotropic Filering modifications, and Medium/Low detail doesn't active AF.

Second thing, if you don't already have Rage3d tweak, I suggest you download the utility at the Rage3D website. You'll need it to change some OpenGL settings that are automatically set at driver default.

Once you have it installed, open the program, and go to the "Rage3DTweak" tab. There, you'll see some menu options in the form of dropdown lists.

VERY IMPORTANT STEP (Enabling Avanced Tweaker mode IS REQUIRED FOR THE FOLLOWING TWEAKS)
You'll want to go to the "Tweaker" option first. Drop down the Tweaker menu, and pick the option "Select tweaker mode", and set it to advanced.

Then, open the OpenGL tab, and change the following settings...

Enable:
-3DNow! (AMD Only)
-AGP Textures
-Heirarchial Z (seems to be especially important)
-Multitexture
-SSE (For AMDXP/Intel)
-SSE2 (For AMD64/Intel; AMDXP chips don't support SSE2)

Disable:
-Force 16bit Textures

I'm not absolutely sure about this, but I think you may need a restart, as these settings are in the registry, and the registry changes don't activate until you reboot. I may be wrong though, but better safe than sorry...

You can check your results by changing the line "seta com_showFPS '0'" to 1 in the config file in the Doom3/base folder.

With the following changes, I felt that my framerate was higher overall, and much more consistent.