Ok, well I see that everyone has suggested Radeon cards, and the 7870 is definitely one of the best choices.
Depending on what game you play, 7850 may suck for you, so just tell us what games you play.
If you want to go with the green team, I would suggest the GTX 660. It's priced between the 7850 and 7870 and
if you play BF3 (that's my game), then the 660 outperforms both Radeon cards. But otherwise, the 660 usually
does in between the two and it's a good choice for the money.