Check my videos on YouTube (link in sig).
I have created a video that shows you exactly how to install XBMC on your ATV2, and then connect it to your NAS for all content etc. I also have a Synology NAS so can confirm it works.
Also, if you an an iPad or iPhone, you can play media from your Synology NAS directly, and then AirPlay this across to your ATV2, all without having to keep your mac switched on. Again, I have created videos on this as well 😉
I don't want to link to them as the mods may think I'm advertising etc, so I will let you browse at your leisure if you wish to do so.