I was under the impression that there is not a single piece of hardware out, that can actually show 120fps? Im sure the difference between 120fps and 60fps is quite noticeable but since i have never actually been able to compare it i wouldn't know.
Rendering it is nothing special, i remember running Q3:A at 333fps.
Many monitors support 120Hz or more, depends on resolution. Some even do 200Hz+. Vsync on and tripple buffer 'hack' to keep the fps high.