Anything that works in iTunes will work on the AppleTV. Whichever computer you set it up with becomes the main source, and as far as I know, the music from that machine must be transferred/synced to the AppleTV in order to play, but movies can be streamed.
You can also connect other computers to the AppleTV, up to 5 I believe, which you can use to stream music/movies to the AppleTV.
Anything that works in iTunes will work on the AppleTV. Whichever computer you set it up with becomes the main source, and as far as I know, the music from that machine must be transferred/synced to the AppleTV in order to play, but movies can be streamed.
I know it can be streamed from computers, but I just never had any luck streaming the source Mac's music. I could only get it to play that music if it was synced. I could very well be wrong though.