If anything the slight overclock the cards perform on themselves, and those done in the guru3d article seem to show the slight boost actually makes performance better at higher resolutions.
I think the 680 will be the card to beat when it comes to multi monitor gaming. My main gripes with AMD's eyefinity was that you needed a dp port on the monitor or a adaptor to get it to run on your current monitors, although the 680 still has a dp port, two dl-dvi and one full size hdmi is an all rounder approach to display technologies, unless your running a vga flatscreen...
OK, history lesson for the few not getting or understanding the 680 -> 560 references
with the release of the 400 series (so 460/470/480) fermi cards the release structure was like this...
march 2010, the 470 and the 480 were released, both had the GF100 gpu in them
July 2010, the 460 was release it had a cut down variation of fermi released as the GF104 gpu
scoot forward a bit
November and December 2010 we saw the release of the 580 and 570, using the GF110
January and May of 2010 gave us the 560 and 560Ti using the GF 114
So now we have the GTX 680, which many thought after the release of the majority of AMD's current graphics lineup, we would see a true to form release of the GK100
(note: nivida gpu's are Geforce Fermi for the 400/500 series, since the 600 is Keplar its GK)
but, instead we get the GK104, which makes everyone look at the previous 104/114 and go 'hey, thats very interesting' then look at the differences between the 100 and the 104 and the 110 and the 114, which is why everyone is looking at the 680 and wondering what nvidia has up its sleeve RE GK100, or if we will see something similar to the G80 or our beloved 8800GTS and 8800 GTX where the 'high end' cards were released with the G80 core and a year later the G92-200 or the 8800GT was released and nvidia quickly churned out the G9X series cards (9X00's) before moving to the revised GT100 series then the new gen GT200 cards.
Prices are very scary atm, i for one was interested to find out, after a phone call with my bro in law from Singapore that the GTX 680's on sale there were going for ~$700 AUD. If anyone remembers the prices of 8800GTX/GTS's when they first dropped, it looks like we are going into a similar price war unless the 7990 drops and gracefully gives us a price point that makes the AMD series compete.
This isnt a complete washout for AMD, if anything i think it will give people reason to pick up a 7950 or 7990 on the cheap if they are looking to upgrade from a 480/470 or a 5870 meanwhile putting pennies away for when the 'bigger' keplar card drops, or amd's 'response'.
I will be watching 7950 prices closely, i think its the better choice from amd's current range...