I just got a new graphiccard (GeForce GT 630) for a couple of days ago.
I'm using Samsung SyncMaster 959NF.
I used ATI Radeon 4650 before i bought the new one.
I used 120hz in-game.
Now i can't with this new graphiccard.
It only runs 75 ingame (by testing -freq XXX)
Uhm… Try some programs to change the number of Hertz. You can find some on famous and useful webpage by satanioN (http://www.satanion.dk/cs/ – section „Hertz“).
Open up your computer and see if there is a switch on the graphic card. I had this little switch there which prevented me getting full 120 hz out of my monitor.
Be sure you use correct cable because not all cables are able put out 120hz.