There is a PSU calculator here, that could help you.
Buy a quality PSU, at least 80% efficient Bronze, like Corsair, Power PC and Cooling, Antec. Please read the reviews before you buy - newegg has extensive customer reviews of the products it sells.
I try not to base my decision on the reviews because a large chunk of bad reviews most likely come from user error because the idiots don't know how to install hardware into a computer >.<
Also, would the single 12v+ rail on the XFX be enough?