At this point in time WoW is not supposed to use more than 2 gigs of RAM per instance. (5x WoWs max would ever use 10 gigs)

http://forums.worldofwarcraft.com/th...1#242589026545
We're not enabling the 3 GB switch on World of Warcraft as its memory management system is built to handle keeping things under 2 GB. It may change in a future version but the current one won't. Only StarCraft II has it.