The best price per performance card at your level is the 7850. I was a little confused on if you were debating 7850 or 7870 though. If you get a 7850, I would get this one. (ASUS)http://www.newegg.com/Product/Product.aspx?Item=N82E16814161406
Here is the 660 I would get this one, one a PPP scale is probably about the same as a 7850 (PNY)
or this one (ZOTAC)http://www.newegg.com/Product/Product.aspx?Item=N82E16814500270
And for a 7870, this one (HIS)http://www.newegg.com/Product/Product.aspx?Item=N82E16814161404
In all honesty, all of them are really good prices. I would probably go for the PNY 660 personally.
You might need to get a better CPU though. Something like an fx 6300 or 8320 would be fine. I wouldn't buy one until you know though. I would also give OCing it a try before you get a new card. But my 7950 was getting a decent bottleneck from that cpu, and none of those gpu's are that much weaker.