I personally don't use multi-monitor gaming, preferring a single large screen (27/30inch 1440p/1600p) over 3+ smaller screens (22-25inch 1050p/1080p/1200p). Therefore, my experiences may not apply, but since this is a forum I will give them anyway
With a single GTX670 FTW 2GB @ 1080p, there is not a single game I have had any issues with completely maxxing out, whether or not the card is (more) overclocked; however, running at >1200Mhz Core and >7000Mhz-Effective VRAM does offer a "smoother" gameplay experience.
With the same card at 1440p, I actually find that frame rates stay about the same, as while at 1080p I tend to enable 4xAA (equivalent) or higher, at 1440p I have only ever used AA a few times, and even then only the lowest setting, as I typically have to strain to see the "jaggies" at that resolution and distance (sitting about 3' away). The same is true for my 21.5" 1080p monitor, which has a pixel pitch even finer than a 27" 1440p or 30" 1600p display, and requires AT MOST 2xAA.
Now, I will absolutely be adding 1-2 more GTX670 FTW cards for SLI, as I like to have "power on tap", and will be adding a dedicated PhsyX card as well as I very much enjoy some of the PhysX enabled games, particularly BL2, and will simply dedicate it to Folding when not gaming (thinking GTX650Ti/550Ti or equivalent/better).
Every chart I've seen on 670's and SLI shows me that they have about the best 2-way GPU scaling of any card available, with 90-95% performance increases being the norm! 3-Way SLI (or CrossFire-X) doesn't scale nearly as well, but with a multi-monitor/very-high-resolution setup could very well be worthwhile. CF-X seems to scale noticeably worse than SLI, but that may just be the results I'VE SEEN and not the reality...
4-way SLI/CF-X is NOT a good idea, and you'd be better off with a PhysX card in terms of performance, even though relatively few games support it.