The folks at The Mac Observer have put together some great guides for upgrading your iPhone.
While you are looking at the pricing guide for an upgrade, check out the Open Letter AT&T released today, giving a little bit of insight on why the upgrade price is so much higher than last year. They are extending courtesy upgrade pricing to users that are available for an upgrade in July, August, or September. This will squelch some of the upset people, but it's not going to please most. On a side note, AT&T has made a good move offering eligible people $200 back if they paid the full price of the iPhone 3G S on pre-order.