It might be game dependant but even something like BF3, which had a clear measurable FPS advantage in SLI, just did not feel substantially better to compared to the cost.
The link of my sig still shows the SLI setup but a single 670 is plenty for 99% of the time at 1080 with 120Hz, even in games suited to 3D like Trine or Dirt.
Can you really not see the benefit of 120hz over 60hz in BF3? Wow it's like night and day to me. No way I could go back to 60hz/screen tearing fest.