The monitor must be capable of at least 120Hz.
See also: http://www.nvidia.de/object...
I don't know whether every monitor works with a higher frequency, but often the designation "Nvidia 3D Vision Ready" is also found as a compatibility note.

It should also be noted that Displayport is used as the connection because other connections are limited in frequency. The GTX970 can use 3D Vision as long as the graphics driver is installed.