Here's the deal
All of these processors will also run Windows XP Professional x86 as well. If you're wanting to use over 4GB of memory (which I would be surprised at, but I have 3 and depending what you want to run, you can fill that space) you will want to go the x64 route. However, you will need to make sure that the other hardware you buy has compatible x64 drivers because Windows Pro x64 requires 64-bit drivers. Most Windows 32-bit programs will run of Pro x64 but you will get to deal with the fun of two "Program Files" directories (one for 32, the other for 64 bit).
Hope that answe