I have 4gm ram and i had the same issue. when i was running around questing everything was fine but as soon as i entered the likes of Stormwind everything started to stutter. I got around this by opening task manager then going to processes, right clicking each of the wow processes and selecting affinity, from here i was able to set all the running wows (5 of them) to use all 4 processors. after doing this all copies ran like a single instance of the game would. i guess this will only really work like this if you have a quad core. give it a try if you do.