If I backup and restore, will I get back all my data, including stuff like the desktop.
Depends entirely on what/how you back up. If you back up the 'desktop' folder you will be able to restore it. Same goes for the others (documents, music, videos, etc.). You cannot however simply backup/restore programs, they must be reinstalled.
Also, is there an option to install 64bit when reinstalling Windows or does the 64bit version of Windows come in a different CD that the 32bit?
Retail discs have both on them and you choose during setup. OEM come 32-bit or 64-bit (only 1) on a disc.