There are several things that might be what you are looking for... The Xeon chips and all P3/P2 you can find mobo's that offer dual or more chips. This will make the system run faster if you are using an OS that will take advantage of it. Win2k Xp or any win server OS from NT4 up. Also Linux supports it.
The other is distrubuted Computing, as was stated above. Although the thought of distrubuted computing is ussually thought of as in massive groups around the world. You can do what is called Clustering at home, but it requires a bit of know how in both networking and process scheduling to understand how it works. The other thing, is you need more then 3 machines I believe. 1 machine to act as the scheduler and pool machine, the other 2 are the process machines. You can do it with 2, but what is the point...
You have to run a Linux varient too as I have never seen aything on a windows system doing this, but I could be wrong.
Something you all might want to read about is Beowulf Clustering.
good luck