It's EASY, very EASY.
Well easy if you know stuff about computers.
But only linux does it.
Beowolf is nice, but that's about taking 2 computers and making them into 1 single computer. Apps have to specificly programmed to use the cluster. It's mostly used for number crunching stuff like DNA research, astrological stuff and so on and so forth.
The EASY way is to use OpenMosix.
Instead of creating a single big machine you are combining the resources of 2 machines into a sort of SMP machine.
What happens is that you have threads. Each proccess is a thread and consumes computer resources, so what OpenMosix does is to migrate the threads from machine to machine to load balance.
So if your playing ut2004 it's not going to help much, since the game is a single thread and can only run on one proccessor at a time, HOWEVER:
If your running a webbrowser, compiling software, ripping cdroms, encoding a movie, running gimp AND playing ut2004 you can setup OpenMosix with a bunch of computers and they will load balance the proccessing power/memory across the entire cluster, buy moving the threads all over the network and they send the results of the proccessing back over the network to the machine were it originated from. All automagicly.
Ever heard of Knoppix?
Well it is a CDROM-based disto. A Linux OS on CD and it doesn't need a harddrive it just runs directly from the CDROM. Good stuff.
Well people have something similar, but modified to run OpenMosix, so what you do is connect a bunch of machines together and pop in the cdroms. They boot up and after a little bit of fiddling you have a OpenMosix cluster.
here is a example of somebody build a 6 motherboard Mosix cluster for serving a super lan Quake3 party.
Openmosix home
On the left hand collumn towards the bottom they have examples of some OpenMosix bootdisks you can use. Even one boots from a floppy.