It is not allowed to upgrade Windows Vista directly from a 32 bit to a 64 bit. You are required to clean install along with all your applications. If you want to go from a 32 bit to a 64 bit, back up all your applications. It is recommended to use the Windows Easy Transfer method. Perform a custom installation of your 64 bit Vista O/S. Your Antivirus and some Hardware Drivers may not work. If you are using a 32 bit Printer, you may want to check with Windows Vista Compatibility Center. Advantages: You can run two different O/S comfortably. Expanded Office perhaps, or run a complex game.
(Pardon me, I didn't see your computer spec's)