I believe I posted this on here before, your backup is stored in the following location in a Windows based PC (This is for XP, if Vista then change document and settings to Users).
C

Documents and Settings\USERNAME\Application Data\Apple Computer\MobileSync\Backup
USERNAME=the user you normally log into Windows with.
In that directory is a GUID value which is the directory that houses all your backup information for iTunes, I recommend taking the backup of the entire directory.
Also you realize you will need to backup more items right? Like the iTunes library xmls, if not your iPhone will be wiped of songs the next time you sync it since that XML will be gone and starting from fresh.
Dixit