I'm about to buy an iPhone 3GS from a friend. He has an extra one because he thought he lost it, got a new one, then found the lost one. I currently have an old skool 3G. Do I need to unlock the 3GS in order to use my SIM card? Can I just go to AT&T and they'll help me out? He's giving it to me for like a $100 bucks, I live in the US, on a family plan (no 2 year contract I believe). I'm willing to do whatever to unlock/jail break it as long as it's cheap, I know jailbreakin is free!