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.