i've underclocked (and volted) my cpu, getting around 700W with 2 7950..
Bad luck we get a lot less with 2 7970 
I've to say it's the power draw from from the wall, i don't know how much power the psu actually gives to my system
maybe i cranked up those cards too much, i do 1400 khash/s with them
and it isn't my final setup, i've too much fans right now