AFAIK the 660 has 2 cores with Hyper Threading (2 physical threads, 2 virtual) and the 760 has 4 cores (4 physical threads). The 660 is also rated a bit higher, at 3.2GHz (or 3.33GHz, I can't remember now), with the 760 rated at 2.8GHz. Also, the 660 has 4MB L3 Cache, with the 760 having 8MB and 1MB L1 Cache.
I'd get the 760 for it's more physical cores and cache and just OC it to 3.2GHz~3.4GHz. You'd get much more performance that from the 660.