I personally found Painkiller to run great, granted I have a 6800 but it's only got 128MB's of ram along w/ my current 512MB, and I had things totally maxed.
Medal of Honor Pacific Assult has some beefy requirements, I cranked things up but I get a hint of a shudder now and then, blame it on the ram.
HL2 is demanding, but it's also very well optimized so it will run on practically any graphics card made this century (okay since 2001 anyway).
So they've got a totally new engine for Unreal Tournament 2k5? Or is it a heavily tweaked one? I'm looking foward to that for sure, 2k4 was a joyride but I found it to run pretty well, even on a 1GHz Mac w/ a Geforce 4MX.
Vampire the Masqurade Bloodlines stressed my PC alot though, I don't even find it too look that great either.