Slowly switching to all Mac Software questions.

For Apple TV, yes your Mac needs to be on if you are streaming your iTunes content to it

 

An NAS should work though, right?

 

It will if you jailbreak using something like atv flash.

 

I suggest running one of your Macs in Windows & OSX mode so you can control how the various media are transferred over in a way that you want.

Generally speaking iTunes throws everything into one big library from which you can pick from to make playlists, which are virtual collections. This allows one song, for example, to reside in more then one collection.

 

The Apple TV 2 streams music from iTunes using Home Sharing. There must be a computer running iTunes active on the network for the Apple TV to be able to stream music and video.

 

As stated you need a machine on the network running iTunes in order to stream to the Apple TV.

I'm not a great fan of jailbreaking devices, keep it simple and stay within the limits Apple set and things tend to just work. Not to say that I don't understand why people do it and there are some great guides on here and other sites should you choose that route.

One option is to look at picking up a cheap G4 machines from eBay or similar to run the iTunes library. I use a Mac Mini Server at work and an old G4 PowerBook at home for streaming my stuff to ATV and AirPlay speakers.

 

No - unfortunately there's not. Even if you've got a NAS box that supports iTunes - like the iomega ones.

It's not enough for the NAS box to support iTunes - its got to do 'iTunes Home Sharing'. Currently no NAS box does that - licensing I guess. Before the updated Time Capsule came out last month there were rumors that it might do it. But it doesn't.

One thing you could consider if you want an iTunes server thats always on and talks to ATV2 is to get a Mac Mini attached to a jumbo hard disk - either network attached or USB attached.