I also found a setting to change that helped me. I was having a problem with the campaign crashing anytime a cinematic was playing. On the opening cinematic, it would always occur right as the dude was picking up the Unreal emblem from the ground. I could get by it by hitting ESC to skip the cinematic, but I like to watch them so I get a sense for what's going on with the back story. Later cinematics would also freeze, so I knew it wasn't just a problem with the opening.

Open UTEngine.ini, which can be found at My Documents\MyGames\Unreal Tournament 3\UTGame\Config. Find the line (I recommend using Ctrl-F) called

PumpWindowMessagesWhenRenderThreadStalled=False and change it to

PumpWindowMessagesWhenRenderThreadStalled=True

After making this change the cinematics played just fine. I could see a brief stutter where it used to crash (and throughout other areas of the cinematic) but it kept right on playing.

Nvidia has released beta drivers to fix UT3 problems...

http://ve3d.ign.com/articles/news/35989/UT3-NVIDIA-Beta-Drivers

"Dec 31, 2007 at 2:15 PM"

OLD Drivers.