You might want to think about a nas to store files on. I bought a 2 tb wd mybook live earlier this week and on a gigabit ethernet connection its been pretty much flawless. I can access my files easily on any pc in the house and i don't have to keep my pc running to watch shows or movies downloaded to it. It seems to work well with the atv flash and xbmc, so for sake of simplicity thats the route i'd take.
I second this. First of all, it's always a plus not having to have the computer running. Second of all, you can also use it as a backup drive. And last but not least - it's a lot easier to access in XBMC