How to Read Power Supply Specs

When shopping for PSU's you will always want to look at the specs on the label. The label states (hopefully accurately) the amperages for each of the voltage rails. On the label you will see the following; 3.3v, 5v, 12v (there may be more than one 12v), 5vSB (stand-by), -12v, and for the older models -5v, and under each, the associated amperage. A good PSU will even show the sustained maximum allowed amperages across all voltages (maximum wattage PSU can deliver). To determine wattage, the formula V x A = W (voltage times amperage equals watts).

As the specs of PC power has changed over the past 10 years or so, the PSU's have (mostly) been designed with newer computer specs in mind. High amperages on the 12v are now the standard, ~25A on the 3.3 and 5v rails is fairly standard and the bulk of the DC output is on the 12v rail.
Older PSU's were designed with the bulk of the power on the 5v rail and little (by today's standards) on the 12v. So an older 350W design may have;
3.3v @ 28A (92.4W) +
5v @ 28A (140W) +
12v @ 20A (240W)
but if we add up that wattage, we see 472W... from a 350W PSU? How does that work... I'll tell you. All of that power is not available at the same time. The label explains that between the 3.3v and 5v rail a maximum of 180W is available - it further breaks down the usage saying the maximum from all + sources can be up to 324W while the negative voltages can deliver 26W for a total of 350W. (the specs were taken from an old PSU I have - not a good PSU but definitely not a bad one either for it's time)
And now a disclaimer about PSU marketing language. The use of "Maximum" on the label is not always bad, the use of the work "Peak" IS bad - in order to claim 'peak' the PSU needs only to run for <2sec before failure. "Sustained" is a better word on a label and higher temperature ratings (40C-50C) are better.
Here are three articles which can explain this stuff far better than I
http://www.playtool.com/pages/psuconnectors/connectors.... Explains history of PSU connectors and what they do
http://www.hardwaresecrets.com/article/181 Very thorough article by Gabriel Torres that explains all there really is to know about PSU's
http://www.jonnyguru.com/forums/showthread.php?t=1036 which is quite similar to the HardwareSecrets article but a little less (IMO) thorough