the GTX 660 Ti is a solid and relatively consistent 10-15% faster than the 7870, while the 7950 is anywhere between a bit faster to a bit slower depending on what benchmarks you favor. Of course when talking about the 7950 the “anything but equal” maxim still applies here, if not more so than with the GTX 670. The GTX 660 Ti is anywhere 50% ahead of the 7950 and 25% behind it, and everywhere in between." http://www.anandtech.com/show/6159/the-geforce-gtx-660-... That is a old review though so i am not sure now.
The 7870 is slightly slower than the 660 ti but should be faster than the 660. I would say with the new drivers the 7870 is very much compatible to the 660ti in most games in terms of performance but the 660ti should still be somewhat faster.