If you've recently bought one of the Korean 'PLS' displays, e.g. QNIX QX2710, you will be happy to know that these should go straight up to 120hz, without any alterations to their hardware.
(i.e. no overlord 2b extreme pcb) At least mine has. Post your results, with the monitor brand, and type (glossy or matte) here, and whether you get any distortion when running at the higher refresh rate.
To overclock, simply open NVIDIA Control Panel, go to Change resolution
, select the correct display and click Customize...
Then click Create Custom Resolution...
Here change the refresh rate to what you want. Click Test
and you will find out whether your monitor is capable of running at that refresh rate. If it isn't, keep dropping it until it works.
You may see distortion at higher refresh rates, so just keep lowering it until you're happy. The distortion isn't permanent.
If your screen seems 'darker' or the colours seem wrong when you're running at a higher refresh rate, play around with some colour settings in NVIDIA control panel (brightness, contrast and gamma most importantly)
I assume this should be just as easy for people with AMD cards, have a look here:
Search for AMD on that page. (ctrl-f)
You can also use evga's pixel clock program. A quick google search will find it.
If anyone finds a way to make games run at 120hz in fullscreen mode, I'd be really happy.Do this at your own risk!