I find this message on news://alt.comp.periphs.videocards.ati:

"If this has been posted before, oh well, here it is again then.  If
not, well enjoy.  It worked fantastically for my 9800 Pro.  I pulled
the info from http://www.megagames.com/.

Wil

Begin pasting...

I think I've found the source of why this game runs comparably slow on
ATI hardware vs. nVidia at the moment, and found a solution to the
problem.

First, open your doom3\base folder. Double-click on the pak000.pk4
file. In the "window can't open this file .. .bla bla" dialog, go on
and associate the file with an app like WinRar. With this file open in
WinRar, go to the glprogs directory in the file. In there you'll find
the shaders. The interaction.vfp file seems to be the main rendering
shader. Altering this shader to output a constant color turns most
objects into that constant color, except for stuff like computer
screens etc.

So double-click the interaction.vfp file to open it (you may have to
associate the .vfp extension with a text editor like Notepad or
WordPad first since we're going to edit the file). Scroll down to the
fragment shader. You'll find these rows:

Code:

PARAM subOne = { -1, -1, -1, -1 };
PARAM scaleTwo = { 2, 2, 2, 2 };

Add this right below them:

Code:

PARAM specExp = { 16, 0, 0, 0 };

Now scroll down to this:

Code:

# perform a dependent table read for the specular falloff
TEX R1, specular, texture[6], 2D;

Comment out that line by adding a "#" to it, and add another line that
will do the same thing with math instead, so it should look like this:

Code:

# perform a dependent table read for the specular falloff
# TEX R1, specular, texture[6], 2D;
POW R1, specular.x, specExp.x;

Save the file and close your text editor. WinRar will ask if you want
to update the file in the archive, select yes. Close WinRar and enjoy
about 40% higher performance in Doom3. Haven't done extensive testing
yet, but my performance went from 34fps in 1280x1024 to 48fps."