Not no electrician but from all I've read, heard, and experienced I can tell you this. If the power supply weighs less then two pounds of butter then it's just crap. You can hardly go wrong with a good dual-fan Antec or Enermax. Their not cheap but they themselves last a long time if not stressed out and will put out good power. I had a bad powersupply once that mad all the large capacitors around the CPU eventually start leaking.
Everytime my roommate turns the heat off I end up running my athlons at 100%. It all works out.
Considering what people spend on cases or video cards.... they're pretty cheap My 550W Enermax cost a little more than the case I put it in. ( I know most people don't need 550W, I got it only after a failed PS and one that blown up. Both were no name brand 300W's so this time I decided to go all out )
<font color=red>Got a silent setup, now I can hear myself thinking.... great silence</font color=red>
Official specs say the positive voltages should be within 5% of their rated values and the negatives within 10%, but with most power supplies the +3.3V and +5.0V will be within 2-3% and the +12V within 5%. But no-load voltages are often outside those ranges, and the computer's voltage measurement hardware isn't the most accurate in the world and usually off by 1-2%.
Very light weight is a sign of bad power supply quality, especially for the supply case, but heavy weight doesn't necessarily indicate good quality, as Leadman/Powmax/
Robotron has shown. For supplies of a given power rating, the size of the transformer located between the two heatsinks can indicate the quality -- larger tends to be better, but this also depends on the frequency because some supplies run at roughly twice the frequency of others, allowing the transformer to be smaller.
Usually power supplies without much ventilation area in their cases are not very good.
Most have tolerances across the rails of 5% or under. The 12v usually allows 10%, but if you're getting 1v differentials, then that psu is hammered dog sh!t and you need to get it out of your case before it fries something. Enermax's don't have tighter tolerances than any of the others, contrary to popular belief. They just use good stuff. There's a few non-Enermax psu's that actually have tighter "official" tolerances (i.e. flux no more than 1-3% on all the rails).
All the computers I maintain, I usually see them flux about .03v on all the rails. I like to tweak the 3.3 to about 3.33 (I probably see this one fluxing all the time, but never morethan .03v). The 5v I try to get around 5.05 (usually don't flux more than .01v), and the 12v rail is always fuxx0r3d. I've never been able to get it under 12.5 without dropping or increasing one of the other rails. Usually runs around 12.6 or 12.7, with the tiny little bitest of fluxing. It's not like I have anything pulling on that damn rail anyways (maybe a total of 5amps of the 14 or more than it's rated for), so why should it?
I've never used an Enermax yet, so I don't know if they have individual pots for each rail or what. That would be very nice, to have individual pots, instead of two that use some kind of resonance to condition the transformers. I've never really figured it out, just messed with them until it evens out where I want. Someone needs to crack open a psu one of these days, and find out just what the hell an Emermax has.
But I've always gotten good resultz from the Leadman "Powmax" series. I have a 300w and 500w from them, and a couple other 300w "powman" generic supplies. All of them work excellent, and perform as close to anything else than I've been able to find specs for on the net. But then, I also tweak the hell out of them too, so that might be the difference. Their QA suxxs a-hole; I *WILL* agree with that...