How to link two (or three) systems to act as one.

I am trying to host a website, but don't have a powerful enough rig to do it. What I do have, however, is two rigs (exactly the same) that could do exactly whats needed for my site. The problem is, I don't know how on earth they can be linked together to work as one system. I know massive webservers have thousands of linked systems, so my question is, can you do the same with 2? or even 3?

System specs:

2X (or 3X):

*(dell optiplex 775 By the way)
* Intel Core 2 Duo CPU E6550 2.33GHz
* 2Gb DDR2 Memory
* 80GB SATA Hard drive
* DVD optical drive
* 8 USB ports
* 1 Ethernet Port
* 1 Serial port
* 1333mhz FSB
* 1 Parallel port