250W would be more than enough if fully working, but something's not right here and I'd agree the PSU is the first suspect. It could however also be an obscure issue with either motherboard or drive too, but first things first, try another PSU and see what happens then.

it used to have a celeron 1.3ghz cpu in it this machine
and it worked fine with that cpu.. i guess the 1.4ghz pIII has put it over the spec for watts?

going to replace now with Model No: ATX-300GU

Try a different drive first, the one you use now could start to make a short.
That would be my first guess, more so if the PSU would trigger it's protection and cut the power suddenly.

well iev got win98SE installed finally
i changed my psu obviously
but i also changed the optical drive
aswell as the hard drive to a 120GB Seagate
i was trying to use a 16GB CF from kingston via a cheapo adapter before
for whatever reason the bios started detecting ti as 8055 mb or so
but i swear i had previously formatted it at 16gb size (coming out to 14gb or so free space??)
anyway im just glad to be up + running

Should I find this concerning? I'd like to someday max out my Dell Optiplex GX150 to a 1.4GHz Tualatin-S CPU, but I don't know if I'll need a new power supply since mine currently has a 250W one. Since most OEM P4s (including the one that my PSU was pulled from) have 200-250W PSUs and those CPUs consume more power than any PIII, I would think that a 250W PSU would be enough for even a 1.4GHz PIII.

For all I know, the OP could've been having problems with his system due to the quality of his PSU since I hear AOpen PSUs aren't that good, but it doesn't hurt to be sure. 😕

I think the OP's original PSU has Fuhjyyu caps in it. Those capacitors are terrible, and the current draw on CD spin-up probably threw the power out of spec and the system shutdown.

You could always crack it open and check if you still have it and care to find out.

Apparently, the Celeron 1300 and Tualatin 1.4 have the same TDP, whether or not they have the same power consumption is up for debate. Either CPU draws little power however, so even if your PSU dies, you can always replace it with a modern one, these systems don't draw nearly as much power as later Athlon XPs (these were like 80W+), so you won't run into issues.