I did such a thing a while back:
I had 2 486 machines, one pentium1 and one pentium 2 connected through a network, all running suse6.3 whit the beowulf software installed. I got it working, and the 'cluster' has rendered a couple of povray images.
The point is, 'normal' programs cant use this parrallell computing power, it must be specially written for it to be able to use all the computers at once..
so, besides a nice learning-purpose, it wasn't very usefull