I have a 2.8Ghz P4 HP PC that I made sure had upgradable slots. Only to find out the powersupply isn't so upgradable. I tried a generic 430W PS and it wouldn't boot
I've put in 2 gigs of PC3200 ram and a nVidia 7600 GS 512 MB video card, plus a second hard drive and in spec'n it out, the 250W HP powersupply should be frying by now, but it's holding steady (crossing fingers).
I have a bud that buys his PC's from a local shop and tells them what he needs and they build it. If it crashes, he takes it back and they get it running again. Overall price, he gets more for his buck than buying off the shelf, plus the face to face contact when something goes wrong.
I'm slowly trying to build my own 3.4ghz P4 and gut this HP, but money and life is always an obsticle