I believe the Wii and DS are on an identical course in Japan, and Mario Galaxy = New Super Mario Bros, in terms of sales. Check dis:
As you can see, New SMB outpaces Sunshine exponentially. Now, how does this relate to the Wii, you might say? As you can see from this next chart, the DS and Wii are on identical sales paths in Japan:
http://www.vgchartz.com/hwcomps.php?cons1=Wii®1=Japan&cons2=DS®2=Japan&cons3=X360®3=Japan&align=1
So, since Wii = DS in Japan, and New SMB = Galaxy, we can guess that sales of Galaxy will be closer to 5 million in Japan alone. Of course, they are NOT the same game, and this is only a comparison, but that gives you an idea of Japanese cravings for Mario platformers. Were they satisfied with New SMB? Or do they want to take the plunge and enjoy Mario Galaxy? I'm guessing they will want Mario Galaxy. However, it will take a while to get to 5 million in sales, especially with the supply-constrained Wii.
My final guesstimate: Halo 3 10 mil, Mario Galaxy 16 mil WW. It will take about 3 years to get to those levels, however. (By that time I will be long gone outta here so nobody will remember, haha!)
