iphone 3g back to 3.1.3

[spyder32]

My brother gave me his iPhone 3g which I jailbroke. Now he want's to use it to get some deal from at&t for the new iPhone 4. I know i have to reset it to factory settings to clean the jailbreak. But what i want to know is how do I get it back to 3.1.3 and re-jailbreak it? I've heard that 4.1 sucks on the 3g and i really like 3.1.3. Please help.

[i.Annie]

You can simply shift+restore to 3.1.3 stock fw to wipe the JB out.

Then when you get it back just JB it with Redsn0w 0.9.4 (point at 3.1.2 ipsw).

[spyder32]

I'm new to this so please excuse my ignorance and thank you for helping, but will this erase everything on my iPhone, Apps, Contacts Etc.? If not, could you explain how to do what you sugested? Thank you very much for your help.