Our 5x 280x rig runs of two of these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817182264It'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.
Thank you, I'll take your advice on that PSU. It's hard to find one with 4 6+2 pins for such a good price.