Actually, 680i has problems all across the board. When using SLI, no sound works, there is constant crackling through the speakers, there have also been reports that the motherboard has been killing off RAM.
Now, the thing with Nvidia and Crossfire is that a motherboard has NO way of tell if there is an nvidia or ATi card present in the pci-express slots. It just gets signals to put on the monitor. That is all. All SLi and Crossfire is done through drivers of the video card. So with a third party chipset maker, in this case Intel, you can use either SLi or Crossfire and it will work. You just need to find the hacked drivers for SLi.