I have a Core2 Duo T7300 2.0GHZ, 512 meg Geforce 8600M and 4 Gigs of Ram (only reads 3). Upgrading to the 2x 2 gigs of ram made a huge difference for me when I started pushing my laptop for multi-boxing. My computer will get pretty hot after opening up 4 copies of wow, so I put it on a laptop fan and it helps. I couldn't do 5 copies of wow comfortably with it, but 4 was fine with extra cooling.

I dropped the idea of using my laptop. I have an old laptop that get's too hot running games and shuts off, so I'm too paranoid to use my good 'work' laptop to run 4/5 copies of wow and risk it over-heating.

I think you'll see a pretty big performance increase by going up to 4 gig of ram and opening only 4 copies of WOW. Just watch the heat output and rig up some kind of cooling system as appropriate.