That is Plenty! I run 4x 7950's on a 850w SeaSonic.
Yeah but that's probably for BTC mining. If you go full throttle LTC mining and use the CPU too you'll probably pull more than 850W with 4 7950s.
I have 2 undervolted MSIs running off a low end Antec 400, but if I tried LTC mining I would be well above 500W just for those 2 cards.