I can tell you for sure 2x GTX680's are a little bit bored at 2560x1440 here.
- Battlefield 3 Multiplayer @ Ultra + MSAAx4 + HBAO -> Never below 60fps, average ~90fps with some dips to ~70-75fps on heavy explosions, smoke, or destruction all combined.
- Crysis 2 @ Ultra DX11 + HD Textures -> Never Below 60fps
- GTA IV @ Very High (single GTX680 as no SLI or CFX support) -> Lowest Ive seen was like 54fps. Anything above 40fps is quite smooth in this game.
- The Witcher 2 @ Ultra (No Ubersampling) -> Hardly see it below 100fps
- Skyrim with lots of mods including 2k Textures and all at Ultra -> Never below 100fps.
Overall the cards perform like damn beasts at 2560x1440 and 2560x1600, to be honest I think that's what GTX680 SLI is really made for, I love them.
I would go for a Dell U3011 over 3 monitores any day, mainly because I hate having bezels between monitors and because I prefer the big resolution on a single monitor. Although it's personal taste and you should choose whatever you prefer of course, only thing I can say is Dell U2711 was the best purchase I have ever done for my computer, coming from a 1080p monitor.
Only game that will really make these cards cry a bit is Crysis 1 and Metro 2033 at full settings, and even then they're playable at really smooth frame rate. At Metro 2033 if you disable MSAAx4 you can get really decent performance and play above 60fps most of the times.
Overall, the cards are way too powerful, and you are definitely wasting their power at 1080p/1200p, grab a 1440p/1600p monitor, you will love it!