Hi!

How did DELL and eMachines get away with small 250W max
rated supplies in P4 3Ghz+ and Athlon 64 3000+ systems?

Dell underrates their power supplies *greatly*. I've never had a
problem with one, and they seem to be pretty well made, in line with
other quality brands.

As an example, the 305-watt rated unit (made by Lite-On) in my
Dimension 8300 is powering two hard drives, 2GB of installed RAM, an
ATI Radeon X1350, two optical drives, floppy drive and a full host of
PCI expansion cards. It has no problems, and has been doing that since
it was new. The air coming out of it is moderately warm.

At first it powered a 3.4GHz P4 Prescott which ran distributed.net
around the clock. Later, I switched to a 2.8GHz Northwood as the
Prescott was good only for putting out heat sufficient to warm a small
country.

The eMachines supplies have two things working against them--the first
being that in some cases they don't have working overvoltage
protection in place. This results in motherboards getting fried. I
haven't seen too many that I could say failed as result of running out
of capacity anyway.

And you could do worse, with a supply that lies about its
capabilities:
http://greyghost.mooo.com/psuthoughts/

William