It's not a problem, but here's the issue - AT&T won't allow an iPhone on their network without a contract, and their networks/technology CAN tell what phone you're using. They MAY be willing to put you on a prepaid iPhone plan, but that's almost as much as the monthly plan anyway.
Unfortunately,
FOR NOW, you're out of luck because the 2G was upgraded to 3.1.3. It CAN be jailbroken, but you would need it to be downgraded first to 3.1.2, and that's not a project for someone technologically impaired.
however, the good news is that within the next few weeks, a new jailbreak should be released for 3.1.3, so it's just a matter now of waiting for a little bit.
If you want to TRY to downgrade your 2G, you can find a few threads about that on these forums that go into great detail doing such.
Good luck with whatever you decide though.