We just had a long thread on this in like June, if not more recently.
Wii sold NSMB just as much as NSMB sold Wii. Without them being tied together it wouldn't have had as great an impact. NSMB had the chance to sell so much because the Wii was already appealing to a wide demographic. Plus the GCN's controller is not very good for 2D platformers anyway.
And if the game was already out on GCN, then the next 2D Mario game on Wii would pull a NSMBU and sell less, as it would probably be too similar. How much less I do not know.
I'd guess GCN hits 35 million at the best case if NSMB released 2002-2004. I don't think it would affect Wii's sales much, no one was basing their decisions to purchase it off of how good/bad the GCN was.
Now, if NSMB was a launch game for GCN perhaps it could have changed the system's whole trajectory... still overshadowed by the PS2, though.
