In my opinion, I'd pick up a second 580. I'm in the same boat, short of buying a GTX 690, a single 680 would not provide a big enough boost to justify the cost of the card ($400-500).
Buy a second GTX 580 for about $300 from Amazon and enjoy a massive performance increase. -- If we lose this freedom of ours, history will record with the greatest astonishment, those who had the most to lose, did the least to prevent its happening.
