The 16-lane (x16) slot has replaced the Accelerated Graphics Port(AGP) on many motherboards and fits a PCIe graphics card. Boards that include the x1 and x2 slots usually have them for other components, such as sound or networking cards. Cheaper motherboards with more budget-oriented chipsets might only go up to a single x8 slot, even if that slot can physically accommodate an x16 card. Meanwhile, “gamer” motherboards will include up to four full x16-size and x16-lane PCI-E slots for maximum GPU compatibility.
The PCI expansion slot was introduced by Intel, but can be found in both PC's and Macs. It displaced previous computer buses (VESA Local Bus and ISA). PCI was then succeeded by the PCI-E or (PCI Express slot), but PCI is still found in most computers because many expansion devices don't need PCI-E capabilities.
It is also possible to get a PCI card that had more PCI expansion slots on it. This would be helpful for a computer that did not have enough PCI expansion slots for further expansions, although most computers would have an ample amount of slots.
When choosing a computer case, it is a good idea to choose a case that will allow for a bigger enough case so as to have a suitable amount of PCI expansion slots. This can be very useful if you want to add some cards later, such as a wireless card, or some more USB slots, or a TV tuner?
PCI Express x16 (graphics): PCI Express x16 slots are used mostly for graphics cards, though they can be used with any PCI Express card. Confusion may arise, however, because not all PCIe x16 slots are true PCIe x16. Occasionaly, you'll see PCIe x16 connectors that are physical slots for accommodating graphics cards, but are actually eight-lane (x8) or even four-lane (x4) electrically.
On some boards, even slots that support true 16-lane PCI Express for graphics may revert to eight lanes if you install a second graphics card into a second PCIe x16 slot on the motherboard. The P67 chipset, for instance, has only 16 total PCIe lanes for graphics. So if you drop in two graphics cards to run in dual GPU mode, each card will have just eight lanes available to it. This situation isn't as bad as it sounds, though, since even eight lanes in a PCIe 2.0- or 3.0-based system delivers plenty of bandwidth for most games