I have a Dell M1750 laptop with the Nidia 7950 video card
I use Vista x64 on all of my machines and I use multiple PC's so I never have lag.
I take an extream hit when running more than one copy of wow on this laptop, at beast I get 30 fps on each copy when running two copies of wow.
I have a desktop that has a 8800gtx and I can run two copies of wow and usually get 60 fps on each copy outside of a city.
It just depends on your video card sometimes.
I usually setup keyclone maximizer to limit FPS on side copies of wow.