As much as I would like to tell you how to do this I am going to be mean and tell you to google.
It don't get easyer than that.
Do you know anything about split procesing? Just b/c you have 4 notes running at 3.2GHZ doesn't mean that you have a 12.8GHZ machine. All you your programs have to be rewritten to use multi nodes. Linux however does support mulit-processors but unless the program is written to split the load then its going to use processor 0 more than 1.
I have a dual pentuim box and looking at grellem it seems as if it takes turns processing data.
I know that there is a cluster knopptix so you could boot all your nodes off a cdrom for testing.
Here is an idea for your cluster. Look into running linux on a Xbox. its 900mhz,10-15 gig hdr,dvd rom,64-128meg memory with upgrades for people who like modding. I thought about using 4 or 5 just for the hell of it. I couldn't write any programs for it but it would just be cool to have.