If you wanted to do hardware boxing you can build 4 additional boxes for way cheaper than 3k since WoW's requirements really aren't that much. Unless you are looking to play maximum details on each box then you'd be spending that kind of money.

However with quads cores getting cheaper and cheaper you could spend the same amount building 1 box to handle them all. Just make sure you have a 64bit OS and at least 6gb or ram. The more Ram the better.