After doing some research, I'd say the following games are guaranteed 10 million sellers:
Game (current sales)
Wii Sports (21.86m)
Wii Play (11.60m)
Super Smash Bros Brawl (4.90m)
Wii Fit (2.55m)
Mario Kart Wii (2.01m)
That makes five 10 million sellers, whereas GameCube couldn't manage even one 10 million seller!
Guaranteed 5 million sellers:
Super Mario Galaxy (6.40m) has a small shot at 10 million, I think.
Mario Party 8 (5.00m)
The Legend of Zelda: Twilight Princess (4.77m)
Mario & Sonic at the Olympic Games (4.08m)
That makes nine 5 million sellers, whereas GameCube had only three 5 million sellers.
What do you think? Any other released games that will sell 5 or 10 million? Guitar Hero 3 maybe? What about yet to be released games? I think the next Zelda is a no-brainer, but what else?
All in all it's pretty obvious the Wii is a much better software mover than the GameCube ever was.