How many hard drives do you plan to install?
If you drop the requirement for RAID, which I think isn't worth the effort for a regular NAS appliance, take a look at the HP Microserver and Dell PowerEdge T20, nothing comes close to these in terms of value and they can make an extremely capable NAS.
http://www.serversplus.com/servers/t...svdel-t203692p
http://www.ebuyer.com/722189-hpe-pro...er-819185-421?
I'm not too hot on media streaming servers, but I keep hearing about the greatness of the Plex server - which can run on* and stream to pretty much anything, I'm sure there are other good options to choose from.
EDIT: Needs an OS to run on, Linux is one option.