Hello ...
Unfortunately you can not reach 120 hz unless you are connected via dual link dvi ...
I also bought a Benq xl2420t, and unfortunately or by connecting via cable or via hdmi adapter hdmi / dvi dual link, never reached the frequency of 120Hz, but still remained at 60hz ... then nvidia 3d vision only works if you connect PC to the monitor via dVI dual link ...
I am forced to sell the monitor: (
But if you buy a 3D TV (including those written in nvidia 3D vision), via hdmi 1.4 and the program that you say, you should do it ...
Asus G74sx-91328v 3D ~ GTX560M 3GB ~ 16GB Ram ~ 750GB 7200rpm ~ SSD Intel 520s 120GB ~