Little recap of my opinions:
Windows XP 32 bits: XP is a much older and lighter OS so 32 bits is just fine unless you use memory heavy applications like CAD. It is also fine for gaming if you don't want/need DirectX 10.
Windows XP 64 bits: I truly believe that all the "64 bits incompatibility horror stories" spawn for XP 64 bits. Unless you really need to use XP 64 bits for some cruel twist of faith, stay away. Not that much support for it.
Windows Vista 32 bits: it will work, if you have it don't feel bad, you are in a much nicer situation than people with 64 bits XP. More than enough for the basic office worker, net surfers, ...
Windows Vista 64 bits: Vista will be faster with more memory and the only way to use more than ~3.5GB or RAM is to go 64 bits so ... it is also necessary if you want/need DirectX 10.