Iv tried to 5 box on my rig ( 7950GT, 4GB Ram, E6600 Core2Duo) and didn't have that much of a problem, i ran all WoW in 800x600 with low grapichs and i capped the slaves to 15 fps, was getting about 45fps in SM and about 15-20fps in Orgrimmar, so i guess that rig is well enough to 5-box :P