my desktop is still a single core (yes.. i know... you guys put my rig to shame...) and i am able to 5 box on it.
i do limp through orgrimmar getting 4 fps on all... but once i get out of the city, i get a consistent 8/25 fps on 2 screens @ 1280x1024

i've got an amd 64 3800+, 4G of memory and an 8800GT OC driving 2x 22"

if you are laggy with only 2... odds are, each one is trying to get maximum fps. (hit ctrl-R to see your fps)

you have 2 things you can do. you can drop your resolution to reduce the number of pixels per screen wow needs to render...
and you can limit your fps by setting your maxfpsbk and maxfps. i would recommend 5/15 to start (5 maxfpsbk ... 15 maxfps)

fyi:
maxfps == maximum frames per second (while in the foreground)

maxfpsbk == maximum frames per second while in the background