I have a laptop with a 19" LCD running as the second monitor. I really wanted yet another screen, then I found out about this gem:

http://synergy2.sourceforge.net/

It is a software solution that forwards the main keyboard and mouse to another system as the mouse moves off the edge of the last screen. Man is this nice! The clipboard works also, so moving small bits of text is a breeze. Because the third monitor is on another system, if the CPU start chugging on something I just move back and start something else. I'm thinking of adding yet another system for four monitors!