2x 5850 (150 W each)
AMD phenom quad core ~100 W TDP750 W antec earthwatts
, the one with 4 rails
I was considering to add a third GPU, but i was not certain if my power supply could handle it. 150*3+100 is already 550W, and that's just the GPU+CPU, not including other accessories.
With underclocked ram it is unlikely your 5850s are pulling 150W and while mining your phenom is certainly not pulling 100W. Best thing is to get a kill-a-watt and measure. I know that sounds like a copout but there are so many variables in play anything else is a rough guestimate at best.