If you set a perfmon condition for 2.5gb you can see that wow hops up there. I only see this when under stupid auction addons race the damn game's addon garbage collection and the memory is take up with buffers that windows is giving it via the wow64 abstraction mentioned below.

If you track the garbage collection in game, the game uses pretty much nothing on the normal 'flying gas can slave setup'. Eventually it will hit 1.5gb, but then you will zone, loot, or what not and it will free up.

My worry on a system with low memory (less than 2gb per wow) is just how much time the game spends in garbage collection and how often.