That amount of nemory is needed if you want to pvp or visit org/sw, but if you're happy just chilling in instances it's nowhere near.
As stated in another thread, i got my Glory of the wotlk hero while playing 3-4 toons on a 2gb box.
Back then each wotlk would use 300-400mb on my settings.
Now ib Cata it seems to be more like 600mb and my crappy system cant handle it