Thank you for the quick reply!
i would be playing on medium setting, how much fps should i expect?
Should be in the 30s, but try it and see. It won't be horrible performance, easier to just try it and see what happens than to speculate. If your PC can run 2 video cards in CrossFire, you can also pick up a second 6770 of the same type and get a nice boost, or spend about $160 on a new faster single card, as a 550 watt PSU would be cutting it close for two cards, unless it's a good quality one.