Hey! Good to see you my friend
These are always the troubles I find with SLi/Crossfire. Do I get another card, or get a new gen, then SLi that one later?
Based on the fact you would have to sell your one 580 and then get two more, I vote get a 680. That gives you the ability to keep your other slots free and have a pretty sweet card that is efficient, and gives you the option to SLi easily if you stick to dual slot. I have 680 SLi right now and run 2560x1600 and have no issues playing anything, and I don't bother OCing them at all. Only thing I see the 680 is weak at is CUDA and I have a 580 for anything I use that for.
Either way you go, I would keep your eyes peeled in shopbot.ca for good 580/680 deals and price match with Memoryexpress.com and get their Uber Pricebeat and get 25% of the difference. Sometimes you can get some pretty sweet deals.
Cant wait to see what you decide!