My system: I5-2500K (OC 4.2 GHz), 6870, 16 gigs ram, 1 HDD, 2 SSDs, on Blu-rqay driv. Running furmark = 350 Watts.
Recommended Quality 450 -> 500 Watt PSU , If you might consider going with Xfire of two 6870, I'd recommend the corsar 650 Watt (more tan you need, but I like to be alittle conserative).
Note : as with any PSU, always check for reviews - PSU selected should always be in the top two tiers.
i am using a seasonic x-400 fanless power supply, with a 6850, max 150watts, amd 905e max 65watts, add 35watts for the computer and you get 250 watts. easily supported by a 400 watt psu... heck, i have run a 560ti max 250watt in that system using prime95 and furmark together and without issues. (except for the heat generated lol)
using a quality psu larger than 400 watts will just be quieter (the psu's fan speed is related to power used). cry once (over the price of the x400) and live in blissful silence! in other words buy the x400...