Our 5x 280x rig runs of two of these: http://www.newegg.com/Product/Product.aspx?Item=N82E16817182264

It's just about the nicest 80Plus Gold 750W PSU for the price. The fact that it's modular is icing on the cake. Two of them will run you ~$220, which is not a bad price for 1500W!

We run the MB, 2 GPUs, and 4 of the risers on one of them. The other one runs the remaining 3 GPUs and 1 of the risers. Total draw from the wall (for both PSUs) is 1450W (stock), and 1250W (undervolted @ same hashrate). If you're not undervolting, going with 2x 800W would be better, just to give yourself some wiggle room.