But its such a PITA to get 64-bit windows installed on a 32-bit kernel.. I think the 2008 3,1 would be better for Windows 7,8, soon 8.1 for 64-bit kernel. While I love to tinker myself I think getting 64-bit kernel based Windows 7 or 8 will take up quite a bit of time.
I did hear about Chimera or Chameleon bootloader which would inject 64-bit capability to boot them in 64-bit.
the only 32bit kernal is OSX's..
I run windows 7 64 and a 2,1 MacBook windows cares not at all about Apples EFI which is the only thing 32 bit on the Mac Pro