I use an old ASUS P8B75-V which has 3 PCI slots. I've actually got 3 of these, two in use and one spare one.
 
The BIOS chips in this board are removable so I bought spares and cloned them using a chip programmer. This means all my motherboards now have the same serial number, and I can restore a backup to either of my DAW PC's without worrying about activation issues. One PC is in my studio, the other in the house and I never run them at the same time.
 
If you are considering a new motherboard, check that the BIOS chips are removable/reprogrammable. That way you can buy another motherboard as a spare with the cloned BIOS chips in it and easily swap them out should you get any issue.
 
M.