I'm 5 boxing on my laptop. It's an HP dv6700.

Duo CPU T5450 1.66 ghz
Vista Home Premium 32 bit
4gb ram
Nvidia GeForce 8400M GS

I have all graphic settings in game on low. My alt toons fps are capped at 20 using keyclone. Sound is disabled on all 5 instances. I noticed a big improvement in performance when I switched back to 1 wow folder. When I used mutiple folders follow would break in cities, the game would run choppier in cities, and the load times for stuff like blimp rides was much much longer.

I get some lag in Org but almost never break follow on my toons. I use keyclone with maximizer. I set it up with one large "window" for the main and 4 small "windows" stacked vertically on the right side of my screen for the followers. I autohide my taskbar and use my entire screen but made sure not to overlap my 5 wows on the screen. PIP switches almost instantly. Generally it runs all 5 pretty well but this might be about the minimum hardware setup.