GTX 670 vs SLI 650 ti vs 7950

GTX 670 is certainly the strongest of the bunch, no question about that.

GTX 650 ti in SLI would bottleneck itself due to the GPU only comes with 1 GB of RAM at most times. I've had readings in Skyrim of 1.80 GB memory used on 1080p.

Radeon 7950 is a viable option as well, if you wanna save some cash, it'll still perform good, but not as good as the GTX 670.