[Blind-sysadmins] boot a 64 bit machine with a 32 bit iso?

Hello, Does anyone know what would happen, if I'll have a ploom of smoke, if I try to boot a 64 bit machine with a 32 bit iso? I've got a machine that came in and can't remember if it's a 32 bit or 64 bit system. It's running xp home and it does have an Athlon64 processor in it single core. I need to back it up prior to reinstalling it, and was wanting to use grml for that, without knowing if it's 32 or 64 bit I didn't want it to have issues. Thanks. Dave.