Needs to run 2 gtx 460's 4GB of 1600mhz DDR3 ram and an OC 4ghz x4 965 (BE so 125w) plus leave a bit of breathing room for full load and fans etc.
I was thinking 800w but psus are probably the bit of a computer I understand the least about.
I'd say that even a 700watt would run it but just to be safe go with a 750-800 and also to future proof it a bit aswell, make sure that its a good name brand like Antec, Corsair or Seasonic, as powersupplys that come with the case are often underpowered....... I'd suggest the Antec Truepower TP-750, Corsair HX750 or the Seasonic M12D 80plus Silver 750watt, Depending on your budget.
Corsair HX650, best series of PSU out there imo. How much breathing room are you after, a 650watt psu would suffice but you'd be drawing a lot of power from it. Just over 400watts and a bit over 500watts while using Furmark. Power consumption would go up a fair bit if you overclock.
Since your on a budget a 650w will do just fine, but if you can swing it a 700-750w PSU would be ideal, especially if you plan to OC the GPUs. I'd recommend Corsair for any PSU you choose. Their 700watt model is a nice buy.