20C? I highly doubt that, even at idle, unless you have a very cold room. I installed that same cooler for a guy with the same CPU (q6600) and I think he was getting about 41-43C at idle average on all cores. I think he got to about 63C at load.
At idle, watercooled and clocked at 3.6, my q6600 is sitting right now at about 33C, and the room is somewhat warm after my machine being on all day. So, that being said, I don't see how you would get those temps...did you see some reviews on that cooler getting those temps? And what were the ambient room temps from those results?
Reseating the cooler (after cleaning and reapplying thermal paste) can give you a better contact between the IHS of the CPU and the cooler, and getting the correct amount of thermal paste will allow for far better heat transfer between the metals.