Well you're not gonna need a lot of graphics power, just a lot of connectors... idk which card is best for that, with three-way SLI you can have atleast 6 screens from one PC but any more than that becomes tricky.
Looking around on the AMD website, looks like the FirePro v8800 supports up to 4 displays, and can be used in xfire. Two of these would get you your 8 displays but I'd contact AMD to be sure, actually what I'd do is contact both nvidia and AMD and ask them which solutions they offer so you can compare, and won't run into any unexpected limitations.