Between me and my friends, we had 2 unused motherboards with 3 pcie slots total. All we needed were better power supplies and gpus. We put one of them in my friend's desktop as an upgrade to his older graphics card so it's not running as fast as the other 3, but it brings our total up to 4 5830's.

$650 invested
1000 Mhash

1.54 MH/$ or $0.65/MH

^^proud of that.