@Sam - I was talking more about how DirectX works and not WoW. It seems logical to me that if DirectX is told to load a texture, it might check to see if it's already in VRAM before it goes looking for it. This seems like it wouldn't matter if you are boxing or not. But I don't know if the graphics for each instance of WoW takes up separate space in VRAM or if it's all in one giant pool.

The only way to know for sure is to use a tool like GPU-Z to test on an Nvidia card. Since I have one, I'll try to test this myself sometime.