looking into NAS

So, I currently have a few external hard drives plugged into a usb hub that is connected to my time capsule. I use these hard drives to store all my media (such as movies and tv shows that i purchase from itunes). The way I currently have it set up, is that i drag media into the external, and then add it to my itunes library, so that itunes can read it directly from the external. I'm now in the market for a new HDD and I'm wondering if NAS would be better/equal/worse than what I have currently. Some of the HDD's that I have been looking into (specifically the western digital my book world edition) are advertising an itunes server feature. I'm not sure what that entails, is that something that would be beneficial to me, or is it basically what I'm doing now? (also curious if itunes server would allow an apple tv to access files from the HDD even if no computer running itunes is on)

Thanks!

Apple TV will only access what it stores internally.