You need a clean install there is no upgrade option for any o/s from 32 to 64 bit the only point in going 64 bit is you can use more than 4 gig ram but you need to ensure there are 64 bit drivers before you do it
Are you going to do a clean install of Windows 7 on a XP machine? Might get lucky with drivers. Could try MFG for drivers to make sure you can install 7. Windows 7 and 8 are really good at picking up drivers.
While we are on that subject,would a usb stick that was used on a 64bit system work on a 32bit system.? The reason I ask this is because I get 'Unknown Device' showing up in the Device Manager and the stick will not work in any of the usb connecters.