If you are worried about going over a download limit then get a package with unlimited downloads, although even the unlimited ones normally have a "fair usage policy" but they are usually set pretty high and most ISPs will only enforce it if you go over it month after month anyhow.
The XBOX is pretty light on download amounts when it comes to playing games on XBox live, but very heavy if you start using it for watching online video streaming (like the SKY player)
to answer the bit where you say "does the packets of information I sue on xbox use my download usage?"
Yes, everysingle thing that you see on a computer screen or hear from the speakers on a computer uses yur allowance, basically if it's using the internet it's counting against your download limit to a greater or lesser extent.
Also, remember you will lose your AOL email address when you leave them, so if you use it start making people aware that yur email is going to change well in advance, I would recomend setting up a free email address that is not linked to your ISP, then if you change ISP's in the future again you don't have to change emails again.