Sorry for the technically juvenile question, but you're help is appreciated. Can anyone tell me if I can use my old sim card in a 3g iphone (assuming it gets unlocked)? I heard that using the old sim crashes programs. Or do I have to get my carrier to change my number over to the SIM card the iPhone came with before getting it unlocked and jailbroken? Does the SIM model matter? Does it change my phone either way?

Thanks in advance !!

Yes, you can use it as normal.

Just make sure you turn off 3G if it is not supported by your network.