Ok, I've notice that some motherboards have two 1x PCI Express and some have 1x and 4x along with two 16X. but what I don't understand is why some boards only run SLI 8X speed each card when in SLI?
Also what is PCI-E 1x and 4x and what are they used for? If someone can clarify this I would appreciate it, Thanks in advance.

p.s
I apologize for the noob questions.

I may not have this 100% right, but here's what I understand:

Just like the speed rating on an optical drive, the X refers to the bandwidth associated with that one slot.

1x < 4x < 16x

If you run a single card that is 16X in one of the 16X slots, you get full bandwidth through that one card.

If you run a pair of 16X cards in the 16X slots and in SLI, each card works on half the bandwidth, to provide half the necessary calculations for a given image. It seems this might be inefficient, but the way I think about it, that gives each card twice as much time to perform the calculation as a single card, thus theoretically doubling the image quality.

The 1X and 4X slots should be able to be used for other PCI-Express form cards (there are or were plans for some PCI-Express sound and NIC cards out there if I'm not totally bass-ackwards and confuddled).