I wish to add something here IF you are attempting to run a single machine and multiple copies of wow your lag will moslty come from hard drive load times specially in cities

i would suggest adding at least 5 hard drives to the machine for a lot better performance 2 raids <stripe> for better performance if just to run 4 wow clients on,

and a single drive for operating system and 1 wow client

as most of you will notice that when your lagging bad your hard drive will be running off the planet.

hope this helps