1. Cheap XFire motherboards will have one PCI-e 16x slot and one PCI-e 4x slot.
2. Moderately price XFire motherboards will have one PCI-e 8x slot and one PCI-e 8x slot. If only one video card is used then that single PCI-e slot will operate at 16x.
3. Expensive XFire motherboards will have two PCI-e 16x slots.
#1. The faster the video cards used the more performance you will loose due the to 2nd PCI-e 4x slot. This is a good solution for probably something like two HD 5770s.
#2. Starting with the HD 5830's, I would probably go with two PCI-e 8x slots. There shouldn't any bottle neck whatsoever, unless you are XFiring two HD 5970s. I could be slight wrong on this since I'm personally not interested in XFire.
#3. If you can afford two HD 5970s and you do not want any bottleneck, then there is no reason to go with option #1 or option #2.
Note: Make sure the motherboard you buy is PCI-e 2.0 or 2.1 compliant. If you buy a mobo that is only PCI-e 1.0 compliant, then bottlenecking becomes a larger issue due to half the bandwidth.