I think you would have to move to a pure HDMI output from the computer in order to get to 120. Although the best thing to do is call samsung. I want to be able to up mine to 240 but it has never been more than 60hz even with BD wise Blu Ray. There is more to it than I remember. It has to a lot to do with what the TV is seeing from Samsungs instructions. I also listen to Leo Laporte the tech guy on sat and sundays and he has TV expert on all the time. Last weekend they were talking about this exact issue. He said the best TV out right now is Plasma Vieras that are 600hz rated, but that 600hz is a bit of a lie. It is capable up to 600hz but he said you will bnever see that because of filtering and output sources. The most you will see out of 600hz is around 120+. Cable companies also do not brodcast signals at more than 60hz right now.
The hz numbers are mostly marketing bullshit from what the guy said. His name is Scott Wilkinson