I use a single 6970 2gb for three monitor eyefinity @4800x1200. I previously used both a single 5870 1gb and dual 5870 1gb. Both were great obviously dual 5870s were better but even with two cards, you are limited to a single card's frame buffer of 1gb. You will definitely see a difference with the single 6970 when it comes to using medium and especially high-res textures, but your dual 5850s have more graphics horsepower than the single 6970.
So, essentially yes and no. AA and AF do eat more vram but also require more processing. I only broke up my 2 x 5870 because the 1gb limitation (I had to use low-res textures in some games...blasphemy!) but more so because a couple of my family needed graphics cards at the time.
My budget only allowed for the single 6970, but overall I'm very happy with its performance. Obviously it does much better on games that aren't nice with XFire. I still get at least 4xMSAA and 4xAF in any game I play. Settings are generally at high but can be as bad as medium. I haven't tried batman arkham titles or Metro though. It'd be pretty awesome to add another 6970 though!