I'm currently running dual monitors, one 120hz and one 60hz without any issues (other than when I want to watch 3D content but that's a different animal ) on a GTX 470. Just recently I was able to pick up a 2nd 120hz monitor for a really nice price.
I plugged in the 2nd 120hx monitor in place of the 60hz one and it immediately made my Windows UI change from the Aero theme to the Win7 Basic theme and created extreme lag in any movements on the screen.
I'm running DVI cables on both monitors (although I don't know if they're dual link or not), and simply unplugging the 2nd 120hz monitor immediately fixes the Aero and lag issues on the screen.
So is it the DVI cable I need to change out or is my GTX470 not able to handle 2 120hz monitors??
What exact card do you have? I have a feeling you have 1 dual link dvi and 1 single link dvi port, meaning you can only get 1 monitor up to 120 Hz. As far as the lag goes, I'm not too sure. What were you doing at that moment? Just at the desktop?