Haven't tried them out yet, so this is mostly for me so that I don't forget.

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1)In game bring down the console (ctrl alt ~)
2)r_gamma 1.3 - The default is 1, try a range from 1.2-1.4, some have said to go as high as 1.8 but that is way too bright and takes away from the atmosphere imho.
3)r_brightness 1.5 - Again the default is 1, try a range up 1.8 to what looks best for you.
4) Turn off AA, turn on vsync on.

To boost performance try this!

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 "XX"
Where XX is try increasing this value in according to how much ram you have.