Go for servers (Score 4, Interesting)

Having done this twice in the past 4 years, my suggestion is to use rack mounted x86 PCs/servers with dual graphics cards. With ATI cards you can go to 8 or 16 monitors per server and as long as you keep a ratio of 1 screen / cpu, you should be fine (capacity wise). Using PCs (a) will allow for easy maintenance and (b) will be easy for others to work on them. PCs are also much easier to upgrade (hardware wise) as they keep the manual effort needed to a minimum. We've done this with PCs and PIs. PIs are a fun project and so far they work well, but you *will* be swearing in the process as you will have to figure out many things, including power, cabling, mounting, etc.