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You don't need either, you either use the 64-bit version (recommended) or the 32-bit version of Windows.
Or are you talking about something else? Boot Camp Assistant is 64-bit btw, but it does not matter for what bit version of Windows you want to install.
 
I want to install windows on my new macbook using bootcamp. For windows 7 do I need 32bit version or 64bit?

For Snow Leopard, it doesn't matter whether you are running the OS with its 32-bit kernel or its 64-bit mode (if you even have a Mac with 64-bit EFI). As for Windows, if you have a Core 2 Duo, then there's no reason not to use 64-bit Windows 7, but you have the choice to use 32-bit if you so fancy.
 
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