I put a 5770 in my 2008 MacPro 2,1 8 core 3Ghz when my old graphics card died because that's all my local Apple store had in stock that would fit... the fan is really noisy and doesn't seem to have sped anything up, so I can't recommend that one.
Just two days ago I slapped in a GT 640, as the 2600XT was getting loud and I wanted 3 displays. The brand is Asus, and it cost $100 and performs perfectly.