PCI-E slots are available in X16, X8 and X4 guise. Even thought the sockets look the same they have a different quantity of active data channels, i.e. lanes. They have 16, 8 or 4 data channel lanes available. X16 is typically used for Video cards that have very high data transfer volumes. X8 and X4 are typically used for cards that do not need as much bandwidth/lanes as the Video card.