I would sell one 580 now and stay with the other one until GK110. Selling it now will give you a better price, as it will be devalued after the GK110 launch.
+1 this is what i would do if i had 2x580s,then again there is nothing out there to really max these cards out,you could always stick with them and wait for maxwell. 580's are good cards,theres no need to change them tbh.
+2 1XGTX580 3GB to GTX680 is a minor upgrade, good but not major in term of performance. GTX680 value will downhill once GK110 is out. This is only an opinion...
buy the 680 like you want but get one with a custom cooler and 4gb of video ram. your 580`s have 1gb more vram than a plain 680 man
I never understood the "issue" with SLi vs having single fastest card. I've always had SLi since 2006 and rarely had any issues. I still have my 3x GTX 580 1.5GB which I do admit I'd like to swap for GTX 680 cards just for the extra juice...though granted I have more than enough power right now...and the other reason I'm broke, so that's a no go.
I have 2 gtx 580s, and I understand what you mean about the noise. I have the Gigabyte cards with the 3 fan cooler. can get pretty loud, but I really dont notice it when im blasting away in BF3. Temps never get above 71c after a few hours of gaming but i have an OC of 900mhz. OC them and they will blast a single gtx 680 out of the water, but like everyone else is saying wait for the single beast of a card to come out then, problem solved and of course if you save up you can get water cooling so less noise and heat. You can even get a different case so there is more air flow so maybe less fan power needed if the cards get more cool air. Also its all about where to place your desktop. I have mine on a seperate desk next to my monitor desk and have moved it as far away from me as possible so noise is really minimal. Sorry to ramble, but even the smallest things can make a huge difference
IMOHFO, I would certainly would: Its very overclockable,less heat,less power consumption, more space in your case,etc.
Get the 680 and overclock it to the max. I love my SLI, but to be honest I would like to have just one card. Problem is, that one card is too slow for my needs.