I'll be using
this picture as a reference since the SMD caps show clearly.
In this next pic, you can count the surface mount capacitors (in the red boxes) above, or sometimes below (or both as is the case with the first slot in this pic) the PCI-E slots. This tells you the maximum number of lanes the slot is capable of.
http://img402.imageshack.us/img402/2999/201107130320.jpg
Some boards use channel switches that convert one x16 slot into two x8 slots when they sense a second card. Sometimes they are used to further split x8 into two x4 slots. It takes four channel switches to convert x16 to x8/x8.
http://img703.imageshack.us/img703/3226/201107130324.jpg
So for this board. we can see that the top slot has sixteen surface mount capacitors, eight above (some hidden) and eight below it. The second blue slot has eight. The channel switches between the first two blue slots switch the top slot to x8 when the second blue slot is populated. The third blue slot has four caps above it making it x4. This is fixed, it won't change. The fourth blue slot has sixteen caps so it's an x16. The channel switches above it convert it to x8 when the last blue slot is populated.
So let's put this into practice:
With one card, you're obviously going to be running just one x16 slot.
With two cards, you will most likely put them into the first and fourth blue slots which gets you two full x16 slots. You could put the cards in the first two blue slots, getting you x8 and x8.
With three cards, you would put them in either the first, second, and fourth blue slots getting you x8, x8, and x16, or put them in the first, fourth, and fifth blue slots, getting you x16, x8, x8.
With four cards, you would use the first, second, fourth, and fifth, getting you four x8 slots.
The only two boards capable of 3-way SLI/Xfire in your link are the UD7 and the Maximus. The Extreme4 can do 3-way Crossfire, but I wouldn't recommend it.
