660 or 660 Ti ? I assume you mean the 660 Ti as they are in the same pirce range.
MSI HD 7950 Twin Frozr 3GB = $310
MSI GTX 660 Ti Twin Frozr 3GB = $310
Id take the 660 Ti....as you can see at the link below, it has a slight lead over the 7950 in fps but a huge difference in frame latency....click the buttons under the graph to switch back and forth.
http://techreport.com/review/23981/radeon-hd-7950-vs-geforce-gtx-660-ti-revisited/11
More numbers (1920 x 1080 hi / AA)
http://uk.hardware.info/reviews/3129/10/nvidia-geforce-gtx-660-review-incl-sli-kepler-for-p179-battlefield-3
AvP - 7950 > 660 Ti > 660
Batman AC - 660Ti > 660 > 7950
BF3 - 660Ti > 660 > 7950
Crysis 2 - 660Ti > 660 > 7950
Dirt 3 - 660Ti > 660 > 7950
Hard Reset - 660Ti > 7950 > 660
Metro 2033 - 7950 > 660 Ti > 660
Skyrim - 7950 > 660 Ti > 660
Shogun 2 - 660Ti > 7950 > 660
Unreal 3 - - 660Ti > 660 > 7950
660 Ti beats 7950 - 7 /10
7950 beats 660Ti - 3/10
660 beats 7950 - 5/10
7950 beats 660 - 5/10
The 660 is $125 cheaper tho and makes a strong case for consideration.