Aren't they bandwidth?
How could that not make a difference, 8x and 16x, are you serious?
..because it matters how much information is being passed.
By this, I mean that when you're only using 4x, 5x, or 6x worth of bandwidth then whether you have that information traveling over an 8x or a 16x buss makes little to no difference in actual performance.
Also, please keep in mind that newer boards will use PCIe 2, which has double the per lane throughput of the earlier standard. So a 16x PCIe1 buss has the same capacity as a newer 8X PCIe2.
As far as crossfire and SLI go, the difference between having two 16x slots and two 8x slots is small, but significant. As far as the results from various reviews on the subject, it just depends on the game, setting, and CPU you are pairing the dual GPU solution with. If you really want to know then you should search for the articles online yourself and go through them. For an AMD build it's worth it to go from a 790X/GX or 890GX board to a 790FX if you plan to use two 5800 series cards. For P55 boards, the gap in price between boards with two 8x slots, and two 16x slots via the NF200 chip, is much wider and starts to go into X58 territory.