recognise your HDD's and files.
You can transfer files back to your C: drive but you will have to reinstall all program files (games, applications) in order to use the program with the new OS.
If you install the x64 bit version first you can run that for around 30 days before you need to activate it. That should give you plenty time to decide if the x64 bit version is better for you and if all your programs and hardware work on that version.
If they don't, start all over again with the x32 bit version and activate that instead. I believe that once activated the key is locked to whichever version you activate the key with, but I could be wrong on that point