Two things worth mentioning, firstly MOST AMD cards only have ONE dual link port, the other isn't dual link which can make more than one 120hz screen a bit of a pain, with any newer AMD card if you're looking to buy more than one monitor I'd recommend DP as the input for 120Hz now(which is a shame as Samsungs cheapest 120hz screen is god awesome, a great price and DVI for 120hz).
Secondly, the 680/7970 trade blows depending on the game, but I both expect bigger speed bumps from AMD(with things like the Nvidia path on BF3 running faster on a 7970 than the AMD path as its purely optimised for extracting VLIW instructions over speed, Nvidia' arch changed but is still 1D, AMD has more to gain here), and in surround resolutions AMD gains pretty much the lead in all situations.
I still am not 100% sure as so few reviews bothered to mention it I'm not sure if the ones that did have it right, but if 680gtx's are running half frame rate on side screens, that means essentially all reviews that put 7970/680gtx on par in surround gaming, witha 7970 you're pushing 3 screens at that fps, with the 680gtx you are effectively pushing TWO screens at that res( 60fps x 3 = 180fps vs 60fps x 1 + 30 fps x 2 = 120fps), its essentially cheating to keep up the performance.
Without seeing one in surround screen setup and without testing a variety of games I have no clue if half frame rate on side screens is a great idea or massively degrades the overall experience. Either way the 7970 is just WAY faster in those types of setups, and "on par" in normal, which puts the win clearly in the 7970 side for me. If its £100 cheaper to boot... well.
Either way, I said it with the 5870, the 6970, the 280gtx, 285gtx, the 480gtx, the 580gtx, these are just bad value cards, the 5850/6950/260/265/470/570gtx's were all much better value cards.
For me you were looking at 10% lower performance for 25-35% lower prices depending on the card, or from the other perspective, a 5870 cost you 50% more, and only offered 10% more performance than a 5850. The fact is that you can barely see the 10% performance and if that isn't enough, then really 5850 xfire would be a hugely better performance increase and again vastly better value, and there are so few setups where 5870xfire would have made any difference over 5850 xfire, because there is so much performance most of the time you're cpu limited anyway.
In other words, the 7970 and 680gtx suck balls, are horrible value and not remotely worth thinking about. 7950 at £300 is so much better value, and frankly 7850 xfire even more again, etc, etc.