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?
The rig I have tentatively spec'd out is as follows:
- MSI 870-G45 AM3 Motherboard
- Sempron 140
-
AMD 6870- 2GB Crucial DDR3 1600
- Rosewill 430w 80+ Certified (no sense in wasting power)
The "out the door" price on this rig would be $430 from NewEgg. Using the ROI calculator over at bitcoinX.com, this seems like a no-brainer purchase.
Thoughts? (and thanks in advance!)

Yea, Radeon AMD 6870. I was suggesting if he wanted to do 2X on that card, he'd need more power.
No, the 8600GTS is too old to be of use in mining at the current difficulty levels. It would be better to scrap them and grab some 5770s at this point to get a decent Mhash rate.