Does that instance of wow run at a greater resolution than your others? A higher res will give you a higher memory footprint and going through Dalaran will boost it even more. I run 5 x WoW on Vista64 with 8 gb ram and when I go to Dalaran my main instance (1600x1200) will reach as high as 1 gb of memory used while my other 4 (800x600) are just over half of that.