I would account for at least 5GB for windows itself... and then, what, like 2 GB or so per game you plan on installing? then maybe a few gigs added on to that for disk file swapping...
games take up a varying amount of space, so 2 GB may not be a proper estimate per game.
EDIT: I wouldnt make the boot camp partition more than 42GB, because then you'll be left with less than 10GB for Mac OS X, and may start seeing performance decreases soon thereafter.