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.
2 x 6800?
I have 2 x 8600GTS on an AMD64 X2....I considered putting it to work mining, but my reading led me to think my rig wouldn't cut it.
What do you think I could mine with it?