Home Sharing Setup (Hardware)

Do not be tempted by NAS devices as they only run pseudo iTunes instances which use old sharing protocols for iTunes which do not work with either AppleTV1 or 2.

You need a PC or Mac to accomplish what you want. I suspect even a humble netbook should be powerful enough to serve media to AppleTV as in general data transfer rates are not that high - hook up a large cheap external to a cheap PC or Mac via USB 2.0, run itunes and change the location of the media folder to the external (assuming thgis computer only has a small hard drive). Use Home Sharing to copy stuff over from the Macbook to the new location (internal/external).

I guess it depends on whether or not you need to use this other computer for anything else - if not a small form factor device could be used without a monitor just on and connected to the network. If possible have it connected to your router via ethernet to reduce wireless traffic if AppleTV is running wireless.

AC

You're going to need something running iTunes, so an NAS won't do. Any new computer will do fine for this such as a mac Mini or Zino HD. Or give a used PC/Mac a new purpose, anything that can run the latest version of iTunes can handle this task fine (i.e. anything made in the last several years).