Three of my 7970's are running at 746,788,788 all on air and temps are 68,72 and 74. Mind you I have another 7970 that will not go over 550 no matter what.  For what is is worth my two 290x's run at 880 each and temps are in the high 70's on air. 6 cards spread out over 3 systems all in cases. Two with sides off, of those one has a small house fan pointed at it (ambient temp around 73c) the other is in the garage. My 290x's are also in a case with the side on it in my lounge, again ambient temps are around 73c.