The gpu you picked is a good one, xfiring it will make it better. The fps will differ but not by much. You may have to go down a few notches on your setting to get above playable fps (30). I personally like to go with 40+. Yes, the 750 watt psu in your case is fine. By the wy if you have money right now or 2 6870s get a single better card. I know you said you'd get it later, but I'm just saying. Good pick in card, good luck!