Look at the manual and spec sheets of your supply. It should provide a load chart that has its efficiency at various points. You want your system wattage at full load to sit at the same number as the supplies highest efficiency. The percentage of power lost to conversion comes out as heat, and heat is what kills components.

A few percent at something like 500w can be the difference of 10-15w of extra heat for the supply to deal with. 75% efficiency from a cheaper 850w supply run at the same 500w would leave you with ~125w of power to cool, most of it concentrated on a few components in the supply.