Important factor that people are forgetting/avoiding mentioning is warranty - factory overclocks are covered. Overclock it yourself and that warranty is void. If that card dies (for any reason) then you won't be covered for a replacement. I'd personally stretch to a GTX670 (they are NICE!) but the GTX660 Ti is a very good card also - not something you'll end up regretting.
I like the 7950 but the two games I am going to be playing the most are both opt for Nvidia. Also I want to be able to enable physx for the extra eye candy in Planetside 2 which Nvidia has the exclusive on. Now just trying to figure out if I should just bit the bullet and go for the 670. That is the question
I just bought the GTX660 TI 2GD5/OC. It out preforms the 7950 out of the box in clock speed.