How Many Boxes In A Decent Beowulf Cluster?

Rick the Red asks: "I'd like to build a mini-supercomputer for our High School (and perhaps another for the Junior High). Given that cost is an issue, but given that effective, hands-on demonstration of the principles of parallel processing and clustered computing is the primary goal, how many nodes would it take to make a reasonable demonstration Beowulf cluster? Two? Eight? Sixteen? Has anyone else here done this, and if so are there resources (lesson plans would be nice) on the net for teachers? Also, has anyone networked a room full of computers such that one minute they're individual PCs available for the usual classroom stuff and the next minute they're a Beowulf cluster? If so, how did you do it? And for the final, far out question: Has anyone ever used VMware to create a Beowulf simulator, clustering virtual PCs running on one physical box?"