I recently acquired a sim card from someone who sold their phone to me. They said it was deactivated and if I wanted to use it, I would have to contact at&t and activate it under my account. The sim card they gave me is an at&t card that says "3g" on it. I am eventually planning on upgrading to a 3g device, most likely the At&t Bold whenever it is released. My question is whether I should continue using my old Cingular sim card from 5 years ago, or switch over to the newer "3g" sim card. What would the advantages of switching to a newer sim card be? Would my old sim work with a 3g device to provide full 3g services?

You're better off using a 3G SIM card in a 3G device. For the time being, if things are working fine there's no advantage to switching SIM's at this point IMO (and added risk when you involve at&t CS).

Thanks for the info. Kinda what I thought but I wasn't completely sure. So when the time comes for me to switch to a 3g device, I will contact at&t and tell them to deactivate my old sim, and activate the new one. Correct?