This model although it touts itself as being crossfire enabled, having 2 PCI-E X16 slots, in fact the manual stipulates that they run as 1 X16 + 1 X4/X1 effectively. Having read about a bit, this would appear to mean that the "X4" restriction is electrical rather than in terms of size.
I am presuming this means that although the 2nd (currently free) slot is X16 (it certainly is in terms of its physical size and would therefore seat a PCI-E gfx card), it will be restricted in respect of bandwith and therefore potentially will take a serious performance hit, especially when using higher end gfx card?
Although this is still crossfire enabled, presumably the fact that the 2nd slot has only a quarter of the bandwith capabilities and therefore is going to cripple performance if I slap in 2 3870 radeons, this makes the whole concept of crossfire on this board a bit daft?
well, seeing as though we are pretty hard pressed to push X8 bandwidth right now anyway... I wouldnt think so. Also, crossfire (and sli both) use the 2nd card to process a part of the image, not the full image. Therefore, the amount of data actually going to the card is less. The arrangement of X16/X4 for crossfire is actually pretty common, most boards I have seen are setup this way. SLI boards until recently have been pretty much limited to X8/X8 operation, with a few exceptions for some mobo's.
4x electrical is going to start bottlenecking on DX10 cards, maybe even the upper echelon DX9 cards as well. I don't have the time to gather a lot of benchies on it, but since we're talking about Crossfire you can compare benchies yourself from reviews like this http://www.overclockersclub.com/reviews/diamond_viper_h... with any of the many newer Crossfire reviews that do it properly on X38 or 790 boards - ie, those boards have two full 16x electrical lanes (at least), so that eliminates the bottlenecks. You can see on that linked review that the board has one 16x electrical and one 4x electrical, and the results of even the 3850 in CF are lower than those of 3850 CF at higher bandwidths.