I'm pretty sure the 4870 would do just fine on an Antec 430 PSU with your CPU. You can find a few of those within your price range on Newegg and it's likely the best GPU your specs can take. But like Sparky suggested, the 4850 is a rock solid performer for ~$100.
I wouldn't really suggest the GTS 250 though. It does perform slightly better on average than the 4850...but the thing costs around that of a 4870. And the 4870 is a much better GPU.