Actually, GTX480 has 1.5Gb VRAM. It makes a big difference in performance IMO.
I too, like AA/AF in games. I can turn up settings (AA/AF/Post processing etc) and still get better performance than 5970 or in quadfire.
Having 1Gb on 5970, it would bottleneck terribly @ hi res/settings. Any attempt to turn on AA/AF/Post Processing, and it would turn into a slideshow.
I believe the 2Gb VRAM on your 5870 makes little/no difference on performance. I think there was an article on hardocp:
Mm, I dunno. It's really setting dependent. Anything that breaks above the VRAM of the card makes framerates nosedive, as the 1GB vs. 2GB GTX460 review shows. Whether this remains 100% true for AMD is not certain, as they have (in the past) had more efficient memory routines. :)