Regardless of the plan you choose for the iPhone 2G, AT&T will probably automatically put you on the iPhone plan anyway. One thing you might want to check out with them is if now that the original contract has elapsed on the 2G (figuring the got it around the time it was released), you may be able to get them to customize your plan. But I've been hearing that as soon as they detect an iPhone on their network under a non-iPhone plan, they automatically upgrade you to the full iPhone plan.
I wish you lock and welcome to the world of the iPhone!!!