Thanks for the answer, uber_g.
As for the others, I don't see myself ever using SLI or Crossfire, so buying a board that supports them means I'm paying a premium that could be used to buy a board with more useful slots, or a better video card, HD, or any other component that I might actually use instead. I understand that both SLI and Crossfire boards will support only one card, but I don't want to pay for a useless extra slot. How many reviews have you seen that show that a single high-end video card outperforms two mid-range cards setup in Crossfire/SLI (the combo of which costs more than the single high-end card)?
Maybe the answer is that I have little choice. The single PCI-e16 slot boards may not support enough features to make them valuable for a high-end system. Anyway, at least there are two to consider. Thx again, uber_g.