A lot of ppeople have been asking m about 64 bit recently. Most are confused by it. Just wanted to point you to a cool tool called securable.exe, download it from http://www.grc.com/securable.htm and use it. It tells you exactly ONE thing, whether your system is 64bit or NOT. If it IS 64 bit than you can run the 64bit versions of Windows 7 (Vista if you are mad) and even Windows XP. Theother benefit is you can also use more than 4GB of RAM.
However if it says 32bit, then I am afraid you are stuck. So either stick to 32bit Windows (whether XP, Vista or 7) and therefore not be able to use more than 4GB RAM, or your other option is to ‘out with the old, in with the new’. Get a new PC and make sure it is 64 bit!
Good luck!