INFO: What is PCI-Express?
There are currently 4 types of PCI Express slots that might commonly be found on a motherboard. They are x1, x4, x8, and x16. Some motherboards have all four available to suit a variety of add-in cards. The design of PCI Express is such that it can accommodate a large range of cards without forcing the motherboard hardware to support full transfer rates. In general, a shorter PCIe card can be used in a longer slot. Some PCIe slots are open-ended which actually permits even longer cards.
PCI Express has an exceptional bandwidth rate. This means that it transfer large amounts of data very quickly. This high bandwidth has allowed Nvidia to use Scalable Link Interface (SLI) technology to permit the running of similar graphics cards simultaneously to enhance gaming performance. ATI has also capitalized on these data rates for their Crossfire technology that will also allow multiple graphics cards to be used together. Some manufacturers are building motherboards that have up to 4 PCIe x16 slots. That permits the linking of 2, 3, or 4 graphics cards!
This motherboard from Intel, the DP55WG, has 5 PCI Express slots as well as 2 PCI slots. Note that PCI cards do not fit in PCI Express slots and vice versa. Note the location of the key notches in the slots. From top to bottom: PCIe x16, PCIe x1, PCIe x1, PCIe x8, PCI (not express), PCIe x4, PCI (not express)