For reasons that I can only assume are totally awesome, NeoGAF user "Michan" has been keeping an eye on the sales numbers coming out of Japan for the Super Mario series and ranking them in the above graphs. It seems to be something that's gone on in different incarnations over the last few years.
The top graph shows sales of the various games one week after New Super Mario Bros. Wii was released. Below are the most recent numbers, four weeks in. Note how many other games it's already stomped on in so short a time, having conquered the castles of half the top fifteen, including every 3D title (also, notice how Super Mario Sunshine doesn't even make the list).
The question is, how far can it go? New Super Mario Bros. on DS managed to make it up to the #2 spot but there are a ton of DS units out there and it may be unreachable. Take a look at a larger version of the most recent chart below and tell me how well you expect it to perform.