Depends, on what your plans is for future upgrades is. If you don't really ever plan to run SLI, then a high quality 550 watt PSU will suite your needs perfectly, as you will have plenty of room for overclocking too. But if you plan on going SLI in the future, a 750-850 watt PSU will be perfect.
For the 550 Watt PSU I recommend the Cooler Master V550
For the 750-850 Watt PSU I recommend the EVGA G2 850W or EVGA G2 750W