Any potential problems mixing x64 and x86 partitions?

Architectures do not have specific file systems. 64 bit Windows can Read NTFS just like a 32 bit Windows can read NTFS, the same is for FAT32 etc. The only thing that might cause you trouble is if you try to use 32 bit executables on x64, not all are compatible.

"If you're trying to dual-boot Windows XP and Windows Vista, don't install them on the same partition. Even if you install the second OS into a different directory from C:\Windows, you'll run into problems with the second OS replacing files in %ProgramFiles%.

If you're replacing Windows XP with Windows Vista, you should be okay. (You may want to reformat C: or at least rename C:\Windows and C:\Program Files before installing, if the installer doesn't do it for you.)

Be aware that if D: is formatted for NTFS and you create a new user account in Windows Vista, it may not have access to all of the files on D: until you change the permissions. This might not be an issue if you use Windows Easy Transfer to migrate your account."