I have all 5 of my accounts on one box. I have a 8800 graphic card, 4g of ram and a dual core 3g. It runs great. I'm not on a high populated server, but i get almost no lag in the major cities (Might want to turn down the setting in the non main toons screen).

Edit:
I just bought my computer just for WOW multi boxing it cost me about $1,600 for everything. If you want i can link all the stuff that i got or send me a pm.