xpd: DONT install Windows 10 unless youre prepared for potential issues - its been good so far, but its not recommended as a full day-to-day OS as yet, if it was, it would be retail and not in preview.

As for 64 vs 32 bit, do a clean install of 64 bit - you will need the DVD/ISO though, Google it and I think MS have them available. Your 32bit key will work for 64bit fine. 



True it's not recommended, but I'm using it as a day to day OS - but with very good backups and a very good technical background.