I wanted to advertise these Atom Motherboards. @1.6GHz
Intel Desktop Board D945GCLF - Overview
they are mini-itx and cost around £35-40 (ex VAT) for board and processor.
1GB sticks of DDR2 are around £12
all they need is a case and these things will happily run XP or can be used as thin or semi thin clients for less than £100.