Yeah there isn't actually a IPS panel that runs at 120Hz out of the box. Those Korean 27" panels can in some cases be overclocked to run at 100+Hz, but are standard at 60Hz.
As said above only the TN panels that can do 120Hz do that running 2D, or 60Hz per eye in 3D. Its just a drop down menu choice in the advanced settings and BAM, 120Hz refresh rate.
And it is so worth it... 120Hz vs 60Hz has a VERY noticable effect on smoothness, everything from gaming to just dragging a window around.
In most games I find there is a setting in the resolution that you have to select to make it run at 120fps, it usually shows the resolution and refresh rate, so in my case I choose 1920x1080@120Hz, and thats it. Game then runs at 120fps(if it can