That power supply could drive two of those cards. And yes, when you overclock your stuff wattage need increases, but the power supply you have chosen is a very good power supply, and as I said, you could even run two cards on it without problems.
I have 3 of those cards (they are really sweet...reasonably fast and very, very cool). One of them is running off a 450w power supply with no problem. Even overclocked the whole system pulls less than 300w when playing games.