NAS/Time Capsule backup system - please help

Yesterday I purchased my first Mac. Previously, I had a Windows 7 PC that I used as a media server to stream files to both my Apple TV and my XBOX 360. My ultimate goal is to create a media network that enables sharing between all my iDevices, my new macbook pro, and my apple TV. I stream a lot of video over my network, it is my primary source of entertainment. This is where the question comes in: should I invest in a NAS, such as a Western digital My Book Live, or should I get an Airport Extreme or Time Capsule? Which of these would function best for my needs? I realize that if I get the extreme, I will have to invest in a separate external hard drive and connect it via USB. If I get the Time Capsule, will I be able to stream media from it, or is the internal hard drive specifically and only for backup? (Note: backup is not extremely important to me - I have an external passport drive that will serve my needs for backup). I essentially need help picking out which network drive will be best for me while simultaneously getting the most bang for my buck. Any advice anyone can provide will be very helpful!

Time Capsule can be used as storage for streaming media but you will quickly fill it and have no room for the backup for which it was designed.