Should be close- http://www.bit-tech.net/hardware/graphics/2010/10/22/at... so you would be using about 500 in gfx cards at max and have 100 left for whatever else. I know I would be a bit worried though. Upgrading to a 750 would be enough but you should never undersell on available power. I'd go for something like an 850 if using xfire or sli no matter what cards.
Well I read and article about ''Crossfirering'' two Radeon HD 6850, and it said a 650W PSU. Though as this one is a Corsair, and that these PSU are always under rated in there wattage, I thought maybe that would be enough.
"Our test system is based on a power hungry Core i7 965 / X58 based. This setup is overclocked to 3.75 GHz. Next to that we have energy saving functions disabled for this motherboard and processor (to ensure consistent benchmark results). On average we are using roughly 50 to 100 Watts more than a standard PC due to higher CPU clock settings, cooling, additional cold cathode lights etc."
The Corsair 600CX will work just fine for Crossfiring 2 of those cards with your system. It is however from Corsair's budget line. That rebate makes it worth it though. If it didn't have the $20 rebate I could have found you a better model for the same price.