Jailbreak questions

I have a few questions regarding my iphone 3g and jailbreaking. I have searched the forums and couldn't get a clear answer.

I have upgraded to the new iphone 4 and I do not plan to jailbreak the iphone 4.

I ordered one of those micro sim card adapters so I can use my micro sim in my iphone 3g.

My iphone 3g was upgraded to ios 4 and then i downgraded it back to 3.1.3. It has never been jailbroken before.

I would now like to jailbreak my iphone 3g and use it as a backup phone and for tethering.

1) What is the best jailbreak software/method to use for my iphone 3g that has been upgraded to ios 4.0 and then downgraded back to 3.1.3?

2) I am on the at&t network so unlocking is not required, right?

Thanks in advance,
bmw

 

Thanks for the suggestions.

I was able to jailbreak it successfully with pwnage tool 3.1.5.

Once I get my mico sim card adapter, I will be tethering in no time!