Well to answer a few questions;
Sounds like a great set up. If you wouldnt mind me asking, why 3 box instead of say only 2 boxing? (I ask only to get more info, no sarcasm intended)
Basically it's to Hardware box as opposed to software on all computers.

I 5x box on a single laptop without issues so I don't think you need to upgrade your computer beside adding ram (my laptop has 4gb and is running vista 64 - I use symlinks for wow shared folders)
What setup do you run (systems spec)? I've seen a few right-ups on 5 boxing from 1 comp, but honestly the WoW maximizer was a pain in the ass to setup for me. I never was able to get keyclone's PiP to work for me. Also would the best bet be to grab just 1 more monitor and quad them on that w/ some sort of PiP/maximixer feature? Any links for "symlink" is that a Vista only deal?

What operating system are you running on the existing comp? That plays a role.
WindowsXP Pro - I'm bitting the bullet and going Vista, but not sure whether I should go 64 pro or not... any suggestions?