http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
The graphics chart shows the GTX 560 at one level above the HD 7770 , so an overclocked HD 7770 would be at the level of the GTX 560. However the overclock is what brings it up a level and the 560 isn't overclocked so that leaves room for it to be overclocked and raise it above the HD 7770.
The GTX 560 would be the card to use.