With that power supply and your current processor, I'd recommend pretty much any mid-range graphics card would work. Something along the lines of an AMD HD5650/4670 or NVidia GTS250 should work for you.
^ I think wolf meant a GT 240. A GTS 250 is in a different class uses too much power . I guess he could have also meant a GTS 450 which is almost as fast but uses less power. Anyway, without knowing more I say the best you can do is a 5670. If you don't need that much power then any cheaper card that doesn't need a PCI-E connector would work just fine.