I run 3 on 1 computer, 2 on the other and it works out great. I have one window per computer maximized, and there is no big loss in frame rate, or increase in cpu temp, etc.

I have around 7 computers in the house, some for my kids, couple for my wife, couple for me. I could easily afford more for a "proper" 5 box setup, but having 5 monitors going at the same time doesn't appeal to me right now. That would mean a new desk, monitor mounts, wiring out the ass, and other fun stuff I'm sure.

But if you have the time and space to do it, by all means go for it