Nothing supports 8x gpu's 6x gpu's, or 5x gpu's for that matter, and there wont be, quad gpu's is all that is supported so far via drivers, the X58 Foxconn Reniasannce has 4 pci-e slots, and core i7, but if you loaded 4 GTX295's into it, you wouldnt be able to run sli, but it would be an awesome folding farm....
i know the k9a2 motherboard(i think thats a 790fx chipset) can support 4 9800gx2's/4 gtx 295's, but its not useful for much more that a folding farm, and i think that only works because the nvidia gpu's are supported by quadro, and the 4870x2's don't work very well/if at all
Yeah, there are alot of boards that have 4 pci-e slots, but as i stated before the best your gonna get is quad sli. Another reason that afaik scaling between 3 and 4 gpu's isnt that much, let alone adding in more.