This isn't for me, but for a fellow gamer I was talking to. He already has the CPU and GPU. We were trying to dissect a performance problem and the dreaded 'bottleneck' term came up ;P I'm just no good at explaining things so I thought I'd jump over here and ask the pros.
An HD4850 or a GTS 250 are probably a proper match for that CPU.
Then again the GTX 285 is very expensive and for roughly the same money you could get an AM3 motherboard, a quad core Athlon II, a few gigs of DDR3 ram and an HD4870 1gb.