What's the brand and model of the 500 Watt PSU?
System Power Supply Requirements for a single GeForce GTX 960
• NVIDIA specifies a minimum of a 400 Watt or greater system power supply. (Minimum system power requirement based on a PC configured with an Intel Core i7 3.2GHz 130 Watt TDP processor.)
• the system power supply must also have a maximum combined +12 Volt continuous current rating of 28 Amps or greater
• the system power supply should also have at least one 75-Watt 6-pin PCI Express supplementary power connector. There are some non-reference design cards that require two 75 Watt 6-pin PCI Express supplementary power connectors (e.g. Gigabyte GTX 960 G1 Gaming 2 GB [GV-N960G1 GAMING-2GD REV.1.0], Inno3D iChill Geforce GTX 960 2GB Ultra) or one 150-Watt 8-pin PCI Express supplementary power connector (e.g. EVGA GTX 960 SSC 2 GB [02G-P4-2966-KR], MSI GTX 960 Gaming OC 2 GB [GTX 960 GAMING 2G]).
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) is the most critical factor.
Overclocking of the CPU and/or GPU(s) may require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.