Well I can let you in on what I do.

First, I am using an white iMac, 2.16C2D, 3gb memory, GF7600GT with 256mb memory. I can run three sessions with no problem, five is a bear. I use Clonekeys to make them talk to each other.

How do I run three copies?

Simple, one copy has sound, the others have it turned off.
All copies have movie features disabled (no hot keys, etc)
The two child copies have graphics set to the minimum, 640x480 or such display in window mode. I arrange it all in a triangle with the leader at top center.

I have three WOW directories, though two of them use links to share data and wtf

I then have one icon per wow client on my bar... just start them up, arrange the windows; seems all copies inherit the primaries window setting - go figure. Sign in to each account then start up clonekeys.