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.http://www.anandtech.com/show/3809/nvidias-geforce-gtx-460-the-200-king/17
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.