Iowa, New Hampshire, and South Carolina don't necessarily pick the eventual winner, but they certainly force also-rans/never-will-bes out, like Iowa will likely spell the end of the campaign for Perry and Bachman.
By Florida at the end of the month, we'll be down to 3, MAYBE 4 candidates (Romney, Santorum, Gingrich, Paul.) Huntsman will drop out after he has his ass handed to him in New Hampshire next week.
If Gingrich or Santorum falter, one's votes will go to the other, making it a Romney/Paul/other. After that, Paul will "suspend" his campaigning because he'll be getting his ass kicked in primary states. If his organization and followers can't manage better than 3rd in a caucus state like Iowa (and probably no better than 3rd in NH and 4th in SC), he'll eventually see the writing on the wall and go the hell away.