1) You wont be able to run SLI on a crossfire board anyway, and all of them give you x16 bandwidth for a single card.
2) There are no Core 2 Duo boards capeable of dual x16 crossfire, only S939 and possibly AM2.
3) All PCIe Gfx cards are x16, barring a small handfull of Matrox ones, but it has been shown that they dont use all this bandwidth. The last example I have seen of such a test was pre-8800 however. I personally think that x8 would have hardly any impact on performance, even on an 8800GTX
4) The 8800GTX will work in x8 mode, if thats what you are asking. I have run multiple monitors in my board with my 8800GTX and my old 7900GT before, and that worked fine. (yes, there are driver conflict issues, you have to use the modded 8800 drivers that support other cards)