I box 4 "alts" on amd duel core 3700, so that should be more then enough for one ... and 5 computers is the way to go.

Because your best bet is to clone disks you might want to make all the motherboards the same, and the video cards similar (8800 for the main 8600 for the others for example). Say get a AM2plus motherboards X 5 and put a 4 core 9000plus on the main and a 4200 (dirt cheap) on the alts ... I have main at 6600, 2nd at 6000 and 4 minors at 3700 all running 4 toons each (the minors on smaller screens with less effects).