I'm still wondering though how you make these rigs profitable with all of the expensive equipment you're using.
What expensive equipment? Motherboard is tiny part of system cost.
3x5970s $350 ea (yeah you can't get them at this price anymore)
Motherboard tray w/ expansion slots: $40 (easier that building a custom frame w/ extenders)
MSI 890FXA-GD70: $180
Sempron 145: $20
2GB of RAM: $10
USB Stick: $5
1200W 80-Plus Gold PSU: $250 (sometimes less need to spot sales & deals)
Total: $1500 for 2.2 GH/s.
Buying el cheapo motherboard would have saved what $60? minus extender so maybe $50 on 2.2 GH/s rig. Thats $0.02 per MH/s.
If $0.02 per MH/s is the difference between profits and losses well you got problems.