I've managed to get some workstation motherboards off EBay and some cheap processors. I use them as desktops and servers as they both have dual six core Opterons.
One is a XW9400, the other a Tyan S2927( which cost £40 ), the S2927 has dual Opteron 2419 which gives 12 cores at a tiny 1.8GHz but at £19 each, still a great start for a virtual machine host. The XW9400 has dual Opteron 8435's which cost me £100 - but again 12 cores at 2.6GHz isn't bad.
I run Ubuntu and use KVM/QEMU for virtual machines and to play with clustering etc.