They are associated with your iTunes account. So when you register your new 3G to the same account and hook it up to your iTunes, it will just download everything to your new device the same way you had it on your other device.
It's no different than say, you having an iPad and your spouse having an iPad on the same account like my wife and I. You can make individual settings for each (such as what syncs automatically if anything at all) but ultimately, anything you've gotten through iTunes is going to be in your iTunes account and available for your new iPad.