I have a rig that's been running 7 months with 3x 7950s getting 1900kh total on a hx750 psu pulling 700w from the wall
psus differ very much, despite the label. Some may even work above reported load for hours, while other die at 80%. There's usually a number on a label per each rail or set of connectors for MAX load.
http://www.techpowerup.com/reviews/NOX/Coolbay_HX_APEX_700W/images/psulabel.jpgcheap brands/ cheap psu always combine the total load as you can see it is a 700w psu(If you look better you can see the load on the 12V is 600 only), mine ax860's can handle 852 alone on the 12V)
I think the way to go psu's are the ax860,ax1200(7 Years pick up warranty), some platinum seasonic's, Lepa g1600(not sure about warranty but 5+ years for sure) and the Evga Supernova 1300 G2(10 years warranty). I dont see any other Psu being a good bang for buck value.
http://www.techpowerup.com/reviews/Corsair/AX860/images/box_rear_close9.jpg
