Just because it's optimised for AMD does not mean they ignored every nVidia card, just you will probably get more performance in that game pound for pound than from an equivalent nVidia card. But it's swings and roundabouts, the difference, depending on how good the engine is, could be 1-2 fps potentially.
In any game where you move around, you want fluid motion, ideally 60fps minimum, or 60fps average but not going much below that. 30fps is acceptable for some games, but again, if you're moving around, 60fps is the sweet spot, for 60Hz anyway.
Back to the OP, a 6970 should be able to handle BF3 maxed out, might be driver issues initially (but I doubt it), so you should be good. It's not like 6970s will be extinct by BF3's release anyway, you can add another one afterwards. And very often with games, the difference between very high and maxed out could be a few fps different, but not noticeable while you game (not when you stand still and examine stuff, which in an FPS and online, you are very unlikely to do except if you are sniping).
