Read the Wikipedia article on 80 Plus. I found that I can get an 80 Plus certified power supply for very little extra money versus a standard power supply at the same performance level. Price them out and find your compromise for performance and price for the wattage of power supply you need. What's nice about 80 Plus is that you can have it only 20% loaded and it is still certified to give you 80% or better efficiency. It also gives you a good idea of what the real wattage that it can support will be since they had to measure efficiency at 100% load. I've seen power supplies I got 5 years ago that fall flat at half of their rated load on the 12v bus but with a method of rating efficiency that takes different load levels I'm a bit more confident in the quality of the actual power supply too.
I've been able to find plenty at my local parts store and online and found 80 Plus Bronze units just about $10 more than a comparable high end unit that wasn't. The cost is plenty worth it to me considering it shouldn't take long to save $10 of power.
80 PLUS - Wikipedia, the free encyclopedia