It's hard to say how Apple will, umm, bake it into the cost, but the 680M is double the price of the 7970m. This is one theory on the apparent A$200 price hike on the high end 27".
My experience might be colored by the 8800 GS's that went into the 2008 iMacs. Those.. were a catastrophe. It's possible things have improved in recent times, although the 330m in my wife's MBP is also rather miserable (and I mean, far more so in OS X than in Win XP).
I feel like you're following me around just to disagree with me, even if I happen to mention that the sky is usually blue and grass is usually green. Aren't you the one who has never previously owned a Mac?