It's always best using 1 GPU if you can, still GTX 680 has pretty bad value compared to the GTX 670 price/performance wise. When doing SLI at 1440p with 2 GTX 670 with only 2 GB vram, you will be bottlenecked in some games. I've experienced 1800 MB usage in skyrim at only 1080p with the GTX 670.
I'd say even though I'm not a fan of AMD you should consider the Radeon 7970 Ghz ed. Simply because it has the right amount of vram to run that resolution which is 3 GB. Also it excels better at higher resolutions because of the high memory bandwidth it has, where the GTX 680 and GTX 670 are lacking behind.
If you were to be playing 1920x1080, then I would only recommend you the GTX 670 hands down. No questions there, but when moving up the ladder of resolution AMD is simply better. I myself am using a GTX 670 as mentioned before at 1920x1080 and it's just amazing.
Radeon 7970 GHz ed. has 384-bit and 288GB/s memory bandwidth.
GTX 680 has has 256-bit and 192.2GB/s memory bandwidth.
You can clearly see that the 7970 has about 33% better bandwidth which means a lot on high resolutions. Take it with a grain of salt, because the two GPUs doesn't use the same architecture which means that the performance difference might be slightly different.
If adaptive vsync, PhysX, Cuda and TXAA is important to you, then go with the GTX 680. If not, then Radeon 7970 GHZ ed. will be better.
At 2560x1440 Yes I own a 7970ghz and its a monster It just chews up games 2 of these monsters would eat up any game on 1440p/1600p. I forgot to mention the amd 7970 since most people prefer Nvidia. Get 2 7970s get the sapphire dual x and just smash games