Depending on your set up, a 4870 or 4890 would both work for an ATi. For nVidia you could go with a 275, but again it depends on your setup because if your CPU isn't powerful enough then it would bottleneck your graphics card.
For Dual Core a 4850 would be fine and not be bottlenecked by your CPU, if you have a Tri/Quad core set up anything above a 4850 / 275 would easily keep you with a good FPS set up.