I use two boxes. I run 4 toons on the secondary machine which is a little Shuttle machine with an E2180 CPU @ 2.8GHz, 2GB of RAM, and a 512MB 8800GT. I'm getting 40 to 60 fps on this machine with 4 running at the same time, so no complaints. The main box is just OP for WoW, so I won't bother mentioning the specs. I use Multiplicity to move the mouse and keyboard action to the other machine, and I wrote my own keyclone-like program to send keystrokes to the other system while playing.

I'm thinking with a little effort I could hook my wife's computer up to the second monitor on my main machine and run 3 boxes, but I'm pretty happy with what I have.