How about pci-e bus speed? Most video cards are x16, but often, you can get motherboards with slots that are mechanically x16 with only 4 or 8 actual pci-e channels. Does the card do a lot of IO or is it a single function with a lot of GPU work using ram on the board?
Thanks
GPUs used for mining can go into x1 slots with extenders, so slot speed is not an issue.
Your biggest issue with a mainboard is having enough space between the PCI-E slots to allow for multiple cards and sufficient cooling. Also, I'd actually think twice about a 430W PSU if you have a desire to do 2X GPUs on that board, especially 2x 6800s.