I started off like you thinking along the lines of if I want to run multiple copies of WoW I will need to use two computers but I started off with two on one PC and just kept adding more and found if I played about with the detail settings I can run 5 comfortably on my PC (upgraded from 4Gb to 8Gb to help).

One other thing I have learned in my short time multi-boxing is that it really is rather simple. I was very apprehensive when starting as I've never played any MMO with the attitude hit attack then wait but I always tried to push all my characters to the limit so I thought if I am working with one character non-stop how much more difficult must it be to do 5, and different classes set up different ways, as well as co-ordinating.

I looked for a step-by-step guide in starting and never really came across one, but you read here about macros, IS, keyclone, switchers, etc. and it all looks daunting. Once I actually started though and bought a copy of keyclone, just messing about over a night or two it's amazing how easy everything becomes and what seems like a black art rapidly loses it's mistique.

In a nutshell my advice: suck it and see and don't be awed by all the guides / options / stickied info you find here.