Look up safestrap for more information. You can only use a touchwiz rom based on the same version as you have now. If you have version 4.3 you need a TW 4.3 ROM
Note to moderator. I wasn't sure where to put this, so if it is in the wrong forum; please move it to the correct one.
P.S. So far I know back up works fine, I don't know about flashing ROMS on it
Update: Restoring using safestrap will soft-brick the phone.