last year i had to purchase a laptop for someone to use for multiboxing. (i work for a famous royal family - so money is not an issue when they want something)

I decided to use an alienware M-17 (8gb ram, 7950m x2 running SLI GFX & 17" screen). this machine was quite happy running 5 accounts at between 35-50fps.

I myself run a Macbook Pro for multiboxing when i'm travelling - depending on foreign internet connection speeds i can get a playable game running between 3-5 accounts at once.

basically you need to get a machine that has a reasonable gfx card and 4-8gb of ram