Overall, the GTX 660 seems to have more raw performance out the box:
However, when it comes to overclocking? Overclocking can be a bit of a wild card, since you have no guarantee that the results will be at all consistent.
Both cards overclock pretty well: it's pretty undeniable that some serious performance gains can be got from the HD 7850
, but the GTX 660 can be overclocked by enough to keep an edge too
At the end of the day? Both cards perform pretty well, and they're probably both alright. If maximum performance is what matters for you, then the GTX 660 might give you a few more percentage points for your money. However, I'd maybe consider other factors such as warranty, noise, card size, power requirements, etc. to help you make your final decision.