OK. Some of you may think me dumber than I am after this question.

What is the difference between a decent power supply and a not so decent one? I've found kind of a lack of information on this and google has proved useless so far.

I know that enermax and antec and some others make awesome PSU's. Given. But, what the heck makes them so special??? What is it that the generic PSU's lack that makes them an issue?

I saw one post recently where it mentioned the actual useable power of the power supply Vs. the overall rating. I.E. a 300 watt PSU could actually have less than 180watts between the rails and can cause issue as components can not draw enough power (I'm not even 100% sure what "the rails" are but I get the general idea).

But, is that it? What is it that makes a 330watt antec that much better than a 450watt generic PSU that is maybe a third to a quarter of the cost?

Also, which of the better PSU's is the best or something resembling the best? Do any of you know any decent links to benchmark testing or a site that rates PSU's?