between the 320 and 640 mb versions of the 8800gts it makes a huge difference the x1950xt had 512 mb so no that's not where the problem is when you look at the x1950xt and 640 version of the 8800gts, since the x1950xt is still 3 times faster in some situations. So AA isn't causing any memory or page flipping since we would see the problem on the x1950xt first.
The X1950XT used in the review you linked had 256mb. But it didn't use any AA, so it might perform much better in parts of the game where AA bottle-necked the game. This review is not even remotely comparable to 5870 vs. 480 in Heaven benchmark where they made an apple to apple comparison.