This post is pretty much just a copy of another question just asked last week.
There are HOW-TOs at << www.tldp.org
>>. Another good site to check out is << openmosix.sourceforge.net >>.
Putting a network together and running software isn't too hard. Unfortunately, unless you are a good programmer, there's not going to be much you can do with a home cluster. Programs have to be specially written to take advantage of multiple processors in a single computer. Imagine how much more it takes to write a program to take advantage of several computers.
For example, let's look at SETI@home. It's a program that can take several hours to several days to compute the results from one work unit. If you could split it up to two computers, it would take a bit more than half the time, because you have to spend a little time assembling the two results at the end. If you have two identical computers clustered, and it takes one 8 hours to compute a result, it could 4.01 hours to compute the same result. In 8.02 hours, you could have two results. But, if you just had the two separate computers each computing separate results, then you would have two results in 8 hours. Maybe even less than 8, if one gets an easier work unit... If you split a work unit in half, and one half is easier, the one computer will be sitting idle while the other computer finishes so they can return one complete result. Otherwise, the cluster nodes would need to be constantly comparing cpu-load and re-splitting the work unit to even it out, and waste even more time.
If you are looking at it as a project, by all means, have at it. But, a cluster of computers isn't going to be a single super-computer for day-to-day programs.
The OpenMosix cluster is a simple cluster... The nodes compare cpu-loads and suffle processes around to the best computer. It doesn't split a single process up, it just finds the computer best able to handle it.