Unless you can find the 64-bit drivers for the Mac hardware somewhere I'd say use the 32-bit. Unless MS has all the drivers. I haven't looked lately.
Drivers are on every Leopard and Snow Leopard DVD since 2008. The MP3,1 uses EFI64, so there is no problem whatsoever. Do not use setup but Bootcamp64.msi from the Drivers/Apple folder to install drivers and make sure you have admin rights and have UAC disabled. For Seven you must use the compatibility option to make the Vista drivers fit before you run Bootcamp64.msi. You right klick to do that.
XP64 is not supported but Vista64 and Seven64 are.
Sounds about right except Snow Leopard wasn't out in 2008 so there might not be some of the newer drivers on it for older hardware.
Max memory for 32 bits on Windows is around 3GB even if you put in more memory unlike the 32bit OS X who's got a higher limit.
kevink2, are you seeing only 2GB on a 2008 mac pro? With 32 bit XP on my 2006 mac pro, windows only see's 2GB (1.99GB, actually... i think).
I ended up doing 64 bit and it worked great till i installed bootcamp 3.0 drivers and was unable to use my apple blue tooth kb and mouse