If it is just for simple media storage and playback, you would do great with what has already been recommended. You might want to look into pooling drives so that the entire system is easy to manage or a RAID 5 in case you don't want to deal with down time while you restore from backups.
For what you mentioned, I would recommend this:
www.lime-technology.com
or
www.flexraid.com
If you don't have a problem with a little down time in the off chance a drive fails, you could just run Windows Home Server 2011 for cheap.