I second the 7770, I got mine for what would have been $105 if I did the rebate (I forgot), but still $125 was a great price for it. Not sure what games you play but mine will just about max out gw2 (I think it's my CPU that pulls it down when I throw shadows on high), it'll max out starcraft 2, WoW of course. Those are all the games that I play.
I would personally recommend the NVIDIA 550ti as I am using one now. I upgraded mainly for Star Trek Online and it runs on MAX settings pretty flawlessly. You may need to upgrade your Power Supply because the 550ti requires 400W or more. I had to upgrade myself to a better PSU.
The GTX 650ti will have very robust performance enhancements, but you must also take into consideration price. The GTX 550ti + GTX 650 both range from $99-$129. The GTX 650ti will range from $180-$200. Quite a price hike.