I think that the cluster battle would be hilariously horrible for the pentiums. It's obvious that the mac cluster would take about 50-500 hours less to hook up than the pentiums, and it's obvious it would do better speedwise. It takes like a week for colleg educated peeps to hook up a linux cluster. A 6th grader can hook up a mac cluster. I forgot the soft you use to do it but i'll look it up 😛
clusters are multiple computers parallel processing information... Basically 16 processors do work faster than 1, so they work together. scalable computing can be many sizes... it's kind of like Seti@home...
As for software, I used launchdenmother by some grad students at UCLA... there's probably more sophisticated software out there by now tho...