We had a 15 screen setup for a while <a href="http://www.flickr.com/photos/richardcunningham/859013515/" rel="nofollow">http://www.flickr.com/photos/richardcunningham/859013515/</a>
tried to run it from 32bit machine, but with 8 graphics chips (on 4 cards) it only left 700MB or so of addressable memory, so it crawled. In the end we ran it on a cluster.