First of all, you only need 25 fps in the screen you are active, set all screens to maxfps 25 and maxfpsbk 10-15 (Not lower then 10, will get follow problems if you do.).

Lotsa comps can work, dual-core / 4gb ddr, 512mb video is pretty bare minimum to run four accounts at decent video settings without having to tune everything to minimum.

A real fps eater is viewing distance. Turn it down. And make sure you set a maxfps to prevent one wow from leeching all of your resources, leaving nothign for the rest.