Jumping from 2G to 3G

[ZeroShift]

I'm brand new here, but have been lurking for a while. I have a few questions about my current situation.

I currently own a 2G iPhone, and would like to switch to a 3G one. If I were to purchase a 3G iPhone through say eBay, would I just be able to use my SIM card from my 2G iPhone without any kind of Jailbreaking/Unlocking?

Also: I'm going to end up selling my 2G once I get my 3G up and running. I want to keep it Jailbroken, as well as unlock it for when I sell it. How should I go about clearing all of my personal data from my current iPhone while keeping the Jailbreak?

[mbrooksr]

depends. what carrier do u have?

[ZeroShift]

I keep forgetting to add that lol

I'm an Official Legit AT&T user.

[brent91055]

As long as the phone is jailbroken, you can use your current ATT sim card. I was using a GoPhone account until I finally deiced to switch to a contract.

[ZeroShift]

Alright cool.
Is it safe to assume that if I switch, I'll actually have 3G instead of EDGE? I've heard some people saying they still only get EDGE, while others say AT&T will eventually detect and force you to purchase a 3G data plan.