A hard drive is not 32bit or 64bit dependant. You OS (e.g Windows) is. So, if you format and reinstall Windows you will lose everything that is present in the drive or partition that you delete. You will have to reinstall programs, too.
So, moving or copying simple files (e.g docs, photos, music, movies, but not programs) to external drive is definitely a good idea, and you will have no problem to access them with a 32 bit or 64 bit Windows. Some few programs may still work even if you don't reinstall them, but most of them will need to be installed again.