PCI Express, or PCIe for short, is a new dedicated high-speed bus that replaces AGP and PCI for add-in Graphics Cards and other devices. PCI Express Slots come in a variety of physical lengths including x1, x2, x4 and x16 as depicted below. Almost all current PCI Express Graphics Cards require a PCIe x16 Slot and will not physically fit into a Slot with a shorter length.