You'll have no trouble with power.  I have a pair of Sapphire 7950's running now, I use CGM, undervolt, slight OC.  Fans at 70% gpu runs 70C and vrm's about 8C higher.  The Vaper-X cooling seems to be good for 3-5C vs other 7950's I have used.  I have a Thermaltake 850 psu, running a OC'd 8350 under water.