A good, solid 600W psu would run at more than $100.
4770 if you don't want to change your psu.
4870/4890 if you could change your psu and have the cash.
5870/5850 if you could really stretch your budget and a new psu.
The 5770 is in limbo for some, as it is more expensive than a 4870, almost performs at the same level as a 4870, though it has DX11 and performance would probably increase with future driver updates. (Though the performance increase could vary from none-at-all to 10~20%)