Try the various combinations on your platform.  I couldn't tell the
difference between utra quality and high quality with my BFG 6800 FT
OC.  No one can be your eyes or judge for you.

If you have a slower graphics card then the lower the quality, the
higher the frame rate and the better the combat gameplay.  Try the
console command

    timedemo demo1 usecache

on the various settings.  This command will yield the FPS during
gameplay and you can decided between the best graphics and the best
gameplay.

IMHO

I find the biggest performance boost is from disabling shadows in the
Advanced options. If you're getting slow framerates, I'd also recommend
disabling anti-aliasing. It's a minor graphics goodie that is a major
performance hog.

I didn't notice too much difference between medium and high quality, except
on certain textures (the texture on the bio-scanner at the beginning of the
game has noticable compression artifacts, even at the medium setting). I'm
guessing that textures are sorted according to frequency of use in the level
and that the least-used textures get compressed the most. At Ultra setting,
all the textures are uncompressed but you need a graphics card with a LOT of
onboard memory or the game will spend a lot of time swapping textures on and
off the card and reducing performance. (So you might notice a hitch in the
game when you pass into a new area that has different textures.)

Doom 3 was deliberately made high-end. It will look great on this years top
of the line hardware but it will look even better on next years hardware.
Ultra quality is meant for graphics cards that have 512 megs of texture
memory. (Those cards don't exist, yet.). If you have a 128 Meg card, you'd
be better of sticking with Medium quality.

-- Nut

Explained here under Main Menu Options
< http://ucguides.savagehelp.com/Doom3/FPSVisuals.htm >

HTH

M.W.H