First, I'm not a Soyo fan. Second, are you sure your power supply will plug into the new board? There are a couple of types of ATX connectors now, as well as 4-pin sockets that I know none of my P3-450's have in them. A new 300W ps shouldn't cost more than $35, though.
Also, HP has brand new machines for $250 after rebate:
http://www.shopping.hp.com/webapp/sh...ion&catLevel=1
Choice of Sempron 3000+ or Celeron 2.8, 512M, 40G drive, CDRW, WinXP Home. I think you even get a free inkjet printer.