Ok, the PCIe xXX is the amount of lanes the slot has. so x16 has 16 lanes, x8 has 8 lanes and so on.
The physical slot on x4, x8, and x16 are all the same.
If it says 2 PCIe x16 slots it means that there are 2 slots on the board that are capable of 16 lanes.
If it says 2 PCIe X16 (and then in brackets says x16, x4 or x8, x8) it means when you have 1 slot being used it uses all 16 lanes for that 1 slot. But if you plug multiple things into the PCIe slots (most common 2 GPU's) it will divide the lanes between the cards. So that (x16, x4) you will have the first GPU running with 16 lanes, and the second GPU running with 4 lanes.
The manual for any board is going to say what the slot is at natively (x16, x8, x4) and how it reacts if in SLI (multiple GPU's)
For GPU's most of the time you are not going to notice a difference between 16x and 8x because GPU's can not fully saturate the bandwidth of the slot, therefore it is hardware limited, not bus limited.
2 GPU's in xfire / SLI on 2 8x slots will not fully saturate the bus.