Here are the requirements for the 7850 and crossfire too:http://www.amd.com/us/products/desktop/graphics/7000/7850/Pages/radeon-7850.aspx#2
Never heard of the VS series from Corsair, but it doesn't seem like it's their high-end models or that it's meant for Crossfire and SLI. They say this unit is for "low-power computer". I think you will be much better off with the TX or HX series. The TX650 should get the job done, or even better if you want a modular unit, the HX650. Or if you want to be in the safe zone, in case you change for two more powerful cards in the future, you could go for 750 or 850 Watt units. But for 2 x 7850, a TX650 or HX650 should be enough.