Without researching it I would venture an educated guess is that more video ram allows you to store more umm texture palets or whatever parts of the world are called, the stuff that is loaded when you zone.

Thus if you are just doing one instance additional video ram is not needed. If you are running 4 instances then you would want more video ram.

7900 is gathering dust on my self, having replaced it with a 7950, then replacing that with a 6600 and replaceing that with 8600 ... lol