You may be right, Sam, I don't know enough about how much data is being pushed through. I don't think that WoW pushes a steady stream of video data from RAM to the video cards, though. If the video cards can store most of the texture data onboard (and they should be able to) then after an initial load when entering a new area or encountering new mobs/players, the performance should smooth out. It may depend on video settings. My dual Radeons (512MB) handle WoW reasonably well, with one card managing four instances of the program. But those are 800x600 and with all settings turned down.

I think that it can be done in a way that allows the game to be playable.