PC Perspective has just published a detailed article that looks at the performance of the newly dropped UT3 demo on both AMD and NVIDIA graphics cards in both single and dual-GPU configurations.

That being said, if you want to blow away your senses and get the top image quality settings with that super-smooth frame rate we all desire, then NVIDIA's 8800 GTX card is the way to go. Our hours of gaming with it were sheer joy and I have no problems calling it the best graphics card for Unreal Tournament 3, period. Runners up include AMD's HD 2900 XT and NVIDIA's 8800 GTS cards, both the 640MB and 320MB versions.

Unreal Tournament 3 Performance Revealed