My suggestion....
1) A refurbished AppleTV (Black) from the AppleStore for �85 -
http://store.apple.com/uk/product/FC572B/A?mco=MjEwNjA2OTQ2) Add aTV Flash Black software for just �12 to enhance the capability of the above (includes PLEX) -
http://www.firecore.com/atvflash-black.This will give you .avi, .divx, .m4v, .mkv, .mov, .mp4, .mpeg, .mpg, .wmv, .xvid playback. This will playback video/audio content from a NAS.
3) Buy a Netgear ReadyNAS Duo enclosure for less than �150.
4) Add the hard-drive(s) of your choice (I now use Seagate Barracuda 2Tb) for less than �80 each.