The update can be seen in the installation instructions that come with the Mac Pros:
Important: You must use a single full-install Windows installation disc (Service Pack 2 is required for Windows XP installations). Do not use an upgrade version of Windows and do not install an earlier version of Windows XP and attempt to update it later to SP2 or later. Use only 32-bit versions of Windows. If you have a Mac Pro introduced in late 2007 or later, you can use a 64-bit version of Windows Vista.