Eventhough the Wii has a much bigger install base than it did when Twilight Princess came out, you can basically expect the game to sell around the same numbers as Twilight Princess/Ocarina. Basically based around how much hype Nintendo builds up and how good the game actually is. Unlike a Mario, Metroid or something like Smash Bros, Zelda is their one title that people actually focus on the content of the game over most everything else. And its sales usually reflect that. The only reason Wind Waker didn't sell as high as the rest is because the GC had a lower install base then the Wii and N64.