I don't know... isn't it called super-sampling or something...
I can only select up to 6x or maybe 8x when using 1 card, I think the two cards take different samples and composite them together in the higher modes - I read up on it when I was looking at which cards to buy 18 months ago.
but as ever, there is some new AA technique out now isn't there? CAA or something it's called, can't remember. I tried temporal but I prefered the normal method..
Anyway, I don't know how it works, I just press the buttons...