After Nvidia picked up PhysX, it was obvious ATI would probably get left out of Nvidia's efforts to spread the love to graphics cards and x86 CPUs (hence ATI hooking up with Havok). (Physics engines, for the uninitiated, are what make your body bounce around with aplomb after getting stuck with a grenade in Halo 3.) But some modders have fixed that and ported PhysX to ATI's Radeon 3800 cards, instantly improving benchmarks.
A Radeon HD 3870 system jumped from an overall 3D Mark Vantage score of P3800 to P4262. Though it won't be available for ATI's latest 4800 cards for a bit, you can download the PhysX enabler this weekend at NGOHQ, which should make your Unreal experience on ATI that much sweeter.