Well, it's a tough call. They perform similarly. Depends on the game. You probably won't see much difference between a 4850 or a 5750. The 5750 is good because it has future-proofing (DX11), while the 4850 is end of the line. But, the 4850 is a good card for the money.
I'd skip the 5670 totally. Go either 4850 or 5750. If you can squeeze out a few more bucks, the 5770 is the way to go.
But, if saved $$ is tops for you, go with the 4850.