An OCed 7950 seems to have a slightly higher potential than an OCed GTX 670. Obviously some games favor one vendor over the other. My selling point is that the cheapest GTX 670 about $80 more (~25% more) the cheapest 7950, when both perform nearly the same.
HD 7950 and 7870 are one tier below the GTX 670
The GTX 660 Ti is in the same tier as the HD 7950 and costs $50 less ($20 less than the VisionTek that I wouldn't touch with a 10 foot pole).
See how in Battlefield 3(a game where Nvidia has an advantage) the 660 Ti beats out the 7950 and the 670 beats them both...http://www.tomshardware.com/reviews/geforce-gtx-660-ti-benchmark-review,3279-4.html
Here is techpowerup's review summaryhttp://www.techpowerup.com/reviews/ASUS/GeForce_GTX_660_Ti_Direct_Cu_II/28.html
The GTX 660 Ti beats the 7950 here too..
Anandtech is a mixed bag... although they focused more on 2560 x 1600 gaming where the 7950 seems to have an advantage.http://www.anandtech.com/show/6159/the-geforce-gtx-660-ti-review/20
The GTX 670's are $50 more than the respectable HD 7950s out there
The GTX 670 is highly overclockable.