Ethernet/wifi cards will work in any pcie slot, can be x1, x4 or even x16 depending on what's available, and what else is hooked into the pcie sub-system. For instance many of the cheaper boards are actually x16/x4, unless you do decide to use the pcie x1_1 slot, in which case that shares bandwidth with the pcie x16_2 slot so you get x16/x2 instead.