Popcorn Hour A110
I've read lots of great things about the Popcorn Hour A110. It handles almost all codecs, is upgradable, can be used as a bit torrent client, it can work from an internal hard drive or also access a NAS. If you put an internal hard drive in the Popcorn Hour it will also work as a NAS - the only problem being that it only takes one disk.
Also have you considered looking at the QNAP range of NAS. They seem very comprehensive, two of my friends have them and can't recommend them enough. I'm in the middle of rationalising my home IT and will be implementing a combination of a Popcorn Hour and QNAP NAS with necessary home powerline networking.
http://www.popcornhour.com/
http://www.qnap.com/