Quote:
Originally posted by gmat
Well i'm an EE so i think i understand them 
Mosfets rated for a higher current simply have a 'broader path'. If you swap a mosfet with a higher wattage one - and drawing the same current of course -, you'll get *exactly* the same heat produced.
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It's not quite that simple. One of the most important characteristics of power MOSFET's is Rds-on. This is the resistance of the current conducting channel when the MOSFET is turned on.
You can get MOSFET's with very low Rds-on, but they are expensive. PC PSU's have a very low profit margin, so the manufacturers aren't going to be putting expensive state of the art parts in them if they can avoid it. I'd almost guarantee that I could find a MOSFET that would consume less power than the ones used in a given PC PSU.
It's not an easy task though. There are a lot of characteristics to be taken into account. (Max drain to source voltage, Peak current rating, Continuous current rating. Gate capacitances, Gate threshold voltages, Miller plateau voltage, Temperature deratings, to name a few.)
Any decent switching powersupply is going to be somewhat tuned to operate with the power MOSFET's designed in. Choosing a replacement would require a lot of knowledge of the switching control circuit to avoid an unstable system. That means you'd have to do a fairly thorough job reverse engineering the power supply, before you could even start. Much too big a PITA for me to be interested in doing it.
Not meaning to pick on you GMAT. It's just a matter of where one's experience lies. Most of the EE's I went to school with are full time programmers these days. I'd be surprised if many of them knew this stuff as well as you.