INFO: What is PCI Express
PCI Express or PCIe is a computer expansion card standard designed to replace older PCI, PCI-X, and AGP. Most modern graphics cards use PCI Express. While graphics cards are the most prevalent add-in card to use PCIe, other devices are adopting this form factor such as network cards, modems, and sound cards. PCIe 4.0 is the latest standard for expansion cards and is backwards compatible with earlier standards. Note that PCIe card and ports are not compatible with PCI or PCI-X cards or ports.
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 transfers 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. AMD 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 multiple 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)