Here is a recent review of a 7870.
http://www.techpowerup.com/reviews/VTX3D/Radeon_HD_7870_XT_Black/
Take a note that this is a review of a particular manufacturer 7870. You should see similar performances across most 7870s.
Also, there is a big difference between the 660 gtx and the 660 ti gtx. For the most part, the 7870 is a little cheaper than the 660 ti. But they have similar performance.