As usual everyone vastly over estimates needed power. Running a 4x5870 rig and a Kill-a-watt monitor I measured my rig to pull about 900Watts from the wall on a gold standard PSU, which means about 785Watts to the system.
So any quality 800 - 850Watt PSU will do you fine, just make sure you get something with either a single rail, or rails that can handle the amperage you draw for the GPUs.
I'm not overestimating, I'm making a recommendation. A little headroom gives extra protection from the downtime caused by having to do an RMA. And a good PSU can last a long time, so getting one that's has a couple hundred Watts extra might save you from buying another one if power requirements are different next year.