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Someone please correct me if I'm wrong but I thought 64-bit versions of Windows only work with the Mac Pro.
 
http://docs.info.apple.com/article.html?artnum=306538

"An authentic, 32-bit Microsoft Windows XP Home Edition or Professional with Service Pack 2 (or later) disc
or
An authentic, 32-bit Microsoft Windows Vista Home Basic, Home Premium, Business, or Ultimate disc"

64 bit XP uses different drivers than 32 bit, and Apple hasn't supplied 64 bit drivers. I would imagine it would be very difficult to hack it to work.
 
Well the Santa Rosa macbook is capable of running a 64 bit OS. The C2D is a 64 bit CPU. Speaking of which, I believe Leopard runs in 64 bit.

Anyway as far as windows x64 on a macbook, all you'd need is drivers.
 
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