According to AMD a good quality 500 watt power supply is sufficient for an overclocked pc equipped with a single Radeon HD 7770 graphics card:http://www.johnnylucky.org/graphic-cards/gpu-power-requirements.html
Both AMD and nVIdia are conservative with their recommendations. Both companies overestimate total pc power requirements because there is a tendency among consumers to purchase very cheap, low quality psu's of questionable performance and reliability.
The OCZ-ZT650W power supply qualified for the PSU Lemon List:http://www.johnnylucky.org/power-supplies/psu-lemon-list.html
Scroll down to the OCZ section and follow the link to the technical review. The review provides detailed information about the psu's problems and issues.