Seriuosly if yeh got a pentium 4 and only 3 gigs of ram I would really suggest yeh install XP

Vista require 2 gigs of ram without paging anything out each instance of wow require up to around 200-500 meg

I really doubt it will be playable once yeh got to outlands

and no yeh dont need a dual/quad core to multi box :P
it just takes some tweaking of the software to be able to do it.