I would say buy a lower wattage power supply. I'm sure a 500 watt would consume far more energy than a 200 watt. More things you add to it the more it will consume, take into account what your going to need on the PC and get one for the specific configuration, or add a little. Most PS's run optimal at 70% capacity, for future refrence.
My mom has a celeron 466 or so mghz PC running on a 90 watt powersupply:-\ and yes, still running. Its like 5 or so years old. Has a 7200 rpm HD, 1 or 2 fans, a CD-ROM, and I put my CD-RW in there... still ran, and I think 2x 128 ram chips(PC100 or PC133). Its good for just going online once and a while, with a 90 watt PS, I wouldn't leave it running 24/7/365.
I had a P2 running w/ 288 ram, CD-RW, 7200 rpm HD, and an old 8gig HD that came with it(not sure of RPM) on a 165 watt PS, ran for 7 years then it kept rebooting, not sure if it was the PS or not, got a new one and still did it. Works fine with XP though<shrug>