I have been able to do a clean install of Vista RTM x32 from a NTFS formatted SATA disk.
Everything works great, but there are a few small problems.
-NVIDIA RC2 graphics drivers are a little sketchy with GPU intensive processes, (IE: 3D Screensavers, Games etc.)
-Connecting to a wireless network has to be done manually as when you scan, it will not find any networks.
My only tip to other users is to extract the Boot Camp Drivers for XP by running the following in Start > Run:
...Macintosh Drivers For Windows XP.exe /A /v.
I am not sure that is right, but it basically lets you extract the Boot Camp drivers because the installer won't work. Then manually install necessary drivers for the Mac Pro.