Well, possible, but it is NOT an in-place simple "Upgrade" in the common sense. To make the 23-bit to 64-bit change you have to do Custom Install that amounts to a completely new installation of the 64-bit Win 7. This means BACK UP YOUR OLD DATA before starting, and it also means re-installing all your application software. For more info, see this M$ website and linked pages.
Since you plan to upgrade your machine also to a larger new hard drive, this may not be all that difficult. Basically you'll be doing a clean install to a new HDD, which is nice, and then transferring old files. Check out the M$ tool called Microsoft Easy Transfer to help with moving data and configuration files after the installation. (Note this tool does NOT move your app software - they do need re-installation.)