A pair of 5850's needs about 18 - 20 amps. The CPU will need about 10 - 12 amps.
A good 650 watt PSU would do, but it may not have all the PCIe power connectors you need.
A Corsair 750TX (60 amp 12 volt rail)http://www.newegg.com/Product/Product.aspx?Item=N82E16817139006
would be plenty.
I'm with englandr. I also tend to overspec PSU's. I try to operate them at 50% - 70% of rated capacity.