I signed up for the 3g when it first came out and they had that out cry about not having unlimited data. I have 6 gigs and it is way over kill. The first month you might use over 500megs but after that I don't think you will come close. Aslong as you stay on wifi for apps and music downloads you should be okay.
I think it seriously the first few months that are killer as you are browsing the web every chance you get and following yourself on googlemaps.. After that when you are just using it for checking emails and finding a quck item 500meg plan should be enough. Just double check to see how much it will cost to go over a meg or 2
