I have an AT&T UVerse Gateway/Router/Access Point myself. I am looking at getting a TC soon. I currently have (2) MBP, (2) ATV2, and an iMac.
I am looking at setting this up the same way as you are. However, you cannot use the TC with your ATV. I am assuming you will want to stream data from your TC to your ATV? You cannot do that. You can stream your iTunes Library but it has to come from a computer.
I currently have a (3TB) Buffallo NAS Drive. It is setup and configured to backup via Time Machine.
When i get my TC, I will use it solely for TM Backups. I will keep my Bufallo NAS setup as my Media Server streaming to my JB ATV2.
As for setting it up with AT&T, I've read that you will want to turn off the Wireless Network on the AT&T Modem and use the TC for your wireless network needs. You should connect a cat5 cable from your AT&T Unit to the WAN Port of the TC. Everything will go through your TC and your AT&T Router is just for the internet connection. The TC is wireless "N" versus the AT&T wireless "G" so your wireless backups should be faster than going through your AT&T Router's Wireless network.
Hope this helps.
<Edited by Host>
Yes what i was planning to do is copy files from my macbook pro onto time capsule and then add them to my itunes library, that way i just have them on my library, keep my laptop on and go to my computers on the atv2 and stream them that way, so i guess i am kind of streaming them from my time capsule via macbook pro onto apple tv2