Unjailbreaking iPhone 3G ( 3.0.1)

Currently my 3g is jailbroken, and I want to unjailbreak it completely.

Can I just upgrade to firmware v4 to undo the jailbreak?

If i do this, will I still keep all my contacts, txt messages, pictures, etc.?


EDIT: it's 3.1.3, not 3.0.1, sorry.

bump...