Couple of days ago I found on the street (near specialized public container for electronic waste) a LGA775 motherboard with Pentium D 820, heatpipe cooling system, 1.5 GB of RAM, gigabit ethernet PCI card and a 160 GB SATA drive. I replaced the power-hungry NetBurst CPU with a spare Pentium Dual-Core E2180 I had in the drawer and it's a perfectly working system.
@Anarkat If I find a case and power supply for similar price, I may make it computer for our computer lair. There are several dozens of old computers, but nothing able to browse the web 🙂