Yeah, I have a XL800GTO too and it starts the game real dark and then leaves the windows desktop all washed out once you leave the game sometimes.
I found that setting your brightness slider in the UT options up 1 notch and then back down often corrected the brightness issue. The washed out look on the desktop after leaving the game was usually only resolved by restarting the PC.
This is part of an email that Chris Dohnal (the maker of the Enhanced OpenGL and D3D9 renderers) replied to me with concerning this gamma correction issue I asked him about when using the enhanced OpenGl renderer:
For my simple put the gamma correction back program, go to
http://cwdohnal.home.mindspring.com/setgamma/. However, note that the reset option won't necessarily reset it back to what you'd want if you adjusted it to anything from the 1.0 default in any video card color controls. Anyway, it's just a simple command line program I use along with an icon to have something to quickly click on to put things back if need to. Also,
shouldn't need to reboot when get stuck with the wrong gamma correction
settings. Just go to wherever your video card's color control settings are,
and then bumping one of the sliders back and forth will usually take care of
putting it back.