Then... as long as the computer is connected to hdmi input #7 on the Denon, and all the hdmi cables are 120Hz compliant, it should work. One problem is, I know computers can be a little picky about hdmi outputs when it comes to connecting them to audio components.
I believe you have to connect the computer to hdmi input #7 to get 120Hz on the X4700H.
And of course make certain all your hdmi cables are 120Hz compliant.
Plus, is the LG C1 120Hz capable? I bet @Sam Posten knows.