Windows Vista x64 Support, Migration,
If your organisation is considering migrating to the x64 side of the server and processor pool, then these few pointers might be of use. Microsoft provides a version of the Windows Pre-installation Environment (Windows PE) specifically for the x64 architecture. However, since x86 and x64 systems have hardware parity, you could also use the 32-bit version of the PE to deploy a 64-bit system. While using unattend.xml, though, Windows PE needs to be architecturespecific: as winnt32.exe must run under the same architecture it is deploying. Since Windows PE does not have a 32-bit compatibility layer, and winnt32.exe under x64 is a 64-bit application, 32-bit Windows requires 32-bit Windows PE. The same applies for 64-bit versions of Windows, which require 64-bit Windows PE.
If you are thinking of upgrading your OS from a 32-bit path to 64-bit, things get a bit difficult. You cannot upgrade from any version of Windows XP (x86 or x64) to Windows Vista x64. Windows Vista on x64 systems is a clean-installation-only product. You might thus consider a migration to 64 only under a fresh hardware upgrade.