I don't like 7950s for two reasons. 1) because you have to run them at intensity 20, they draw more electricity than 7970s. 2)because of high intensity, I found them to be less stable and not as efficient as popular consensus claims.  Moreover, I had the standard Sapphire 7950s (the ones advertised on coinchoose), and those fans are as loud as a industrial vacuum cleaner.  So, if you don't have a basement or garage, or just some separate space for your rig, then it can get really annoying.  I have twelve 7970s, both Gigabyte and Sapphire Vapor-X.  The Vapor-X are awesome and seem to be most stable.  That's not to say the Giga's are bad.  It's just that the Vapor-X gpus have worked pretty much flawlessly.  The Giga's have had a couple hiccups along the way, but nothing serious.  But the Giga's were $400 usd from Amazon and I'm a prime member and I get 764kh on them.  The Vapor-X were $450 and I also get 764kh on them.  A quad rig pulls 1300 watts, without touching voltage. 

-Merc