Basically the socket you are looking at is limited to 16 threads devoted to the graphics card on the motherboard. so if you will notice on all of the motherboards with socket 1156, they will have one PCIe v2.0 slot at 16x and some will have a second that has either a PCIe v2.0 4x or 8x slot. I would chose one with a second slot capable of running at 8x.
What all of this means is what "saint19" said above, when running a second card in crossfire, it will run at either 4x or 8x. In the case of boards that run 8x, your primary card will also run at 8x. Tomshardware has done an article recently about the effects of running crossfire at 8x8x instead of 16x16x and showed that there was negligible difference between the two under most conditions.
oh and the "x" is basically referring to the bandwidth of the slot.