You have the monitor already? For 120fps, you will need to CF/SLI. I suggest two GTX 670s.
I don't have the monitor yet but I am planning on getting the Samsung S23A700D for $290 on Newegg. Would the games I listed really need that much graphics power to get 120 fps? I don't expect to get it at ultra settings, I would be perfectly fine playing on medium-low.