Since you have 3 slots, you might as well buy 3 cards to maximize because you are going to have to pay quite a bit for a new PSU. 

I recommend this PSU if you decide to update to 3 cards later but a 1000W or 850W PSU should definitely work if you are only using two cards.

http://www.corsair.com/en-us/ax1200i-digital-atx-power-supply-1200-watt-80-plus-platinum-certified-modular-psu-na-refurbished