I opened up my mom's Gateway P4 1.5Ghz the other day, just to take a look at it. It's running on a 160W power supply. I have never even *heard* of a 160W psu, but I looked up the model online and I don't see any indication that it's not what it says. It's a Newton Power NPS-160CB.
My 1.2Ghz athlon, which is roughly the same speed, has already burned out a 300W psu. Which is what one would expect. Are P4's that much less power-hungry, or is this PSU lying?