Why are you worrying about efficiency while wanting at least 500W capacity? A power-hungry high-performance system with an 80-85%-efficient supply will still use more overall power than a slower system with a supply of low (60-70%) efficiency.
Very, very few computers ever consume anything close to 500W. Typical draw is less than half as much, meaning a 350-400W supply will almost always be more than sufficient, provided it's honestly rated, but many aren't. Some exceptions are Fortron, Antec, Zalman, Seasonic, and PC Power & Cooling, with Fortron being the best bargain among these and often remarkable value.
Switching to an LCD monitor will probably help cut energy consumption better than any other measure.