Based off of this, you only need 469W for two 5850's with 15% capacitor aging accounted for, so 550W should be just fine: http://extreme.outervision.com/psucalculatorlite.jsp
I'd get a 600W for that job, but overkill is my preference.
I have an 850W PSU and based off of my Kill-A-Watt, I never exceed 570W. Of course, I plan to replace my PhysX 8800GT with a second GTX 470, then I'll get closer to 700W and can put the PhysX card back in to get closer to 850W. And I overclock it all, which you won't want to do if you crossfire.