You could not have asked a better question, as I have pretty much the identical setup right here, on the PC I'm replying to your question on. I run the same Samsung S27A950D monitor, and maintaining 120 FPS in the latest games is hard on any system.
A single Radeon HD 7970 will be ok, but you won't be pushing 120 FPS in games like Battlefield 3 or Crysis 3. I would definitely, without a doubt, recommend the CrossFire HD 7950 solution. I take it your motherboard is CrossFire compatible, and your power supply has enough juice to run the two GPUs.
You won't be sorry with that setup, and you should push very close, if not over 120 FPS in most games. Crysis 3 is hard on systems, but if you scale the graphics down a little (no anti-aliasing, etc) then you should be fine. Enjoy!