600W is plenty for your system. The only reason you'd want any more would be if you know you'll be doing SLI in the future.
I would suggest you consider a different PSU though. The CX lineup is a good entry level PSU, but it isn't designed for high end gaming machines, especially ones that will be overclocked. It has come cheap capacitors in its construction.
Good price right now on this high quality PSUXFX 650W PRO650W Core Edition $49.99http://pcpartpicker.com/part/xfx-power-supply-p1650snlb9