There are also some 790FX and 890FX boards that come with 4 PCIE slots.  Here's a cheap one:

http://ncix.com/products/?sku=58759&vpn=890FXA-GD65&manufacture=MSI%2FMicroStar

I have MSI's 790FX board.  Something you may want to consider is cooling: I've found that having more than two in a box gets hot quickly.  I needed a household fan blowing on em constantly or they would crash.  So I've settled on two to a box now.

EDIT: whoops, that one has 4 PCIE-1 slots... here's the one I was thinking of...

http://ncix.com/products/?sku=52498&vpn=890FXA-GD70&manufacture=MSI%2FMicroStar