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[necrimus]

You can use one copy of WoW for all accounts. I would do it this way for a while and get the hang of things before you look into some of the other options like Smylinking data or using SSD's or Raid. The WTF files will keep your UI settings separate. The only problem to look out for is that the current pwnboxer doesn't handle battle.net accounts just right yet so you have to be sure to select your account from the drop down menu each time you load the games. Not a horrible problem and should be fixed in the next version.