Back up everything in itunes before you jailbreak and you will be fine. If you have an iPhone I would assume you use itunes for your music so you won't ever lose any of it and itunes also backs up all of your contacts, apps, pictures, etc. IMO, upgrading to a new version of the OS is usally a waste of time unless it's something like iOS 5 where they made a lot of fundmental changes to the system. It seems to me like they release these updates just to get people to un-jailbreak their phones.