A 670/680 gives ~60 fps @1080 ultra on BF3.
If you really need 120 fps, then yes, a sli setup might be needed.
But in my opinion it's not worth it, an ips with 5ms is still good for those games, unless you're a professional.
Well if you're into editing the i7 is useful, if it's for gaming, the i5 is the best choice.
Also check if that ram isn't that expensive usually for 1866 speeds, you'll pay too much, a dual channel kit with 16GB ram rated at 1600mhz is what you need.
As far as 120Hz monitors go, I've heard bad things about the acer monitor on the gaming circuit. Most of the people I play against use the Asus or Samsung monitors - I personally have a BenQ and can't recommend it enough; it's got a few extras up its sleeve that give an advantage, while being perfectly legal in tournaments. (Black equalization, for example.)