There are a few upnp/dlna servers for Linux although most don't seem to work very well (or at all) with the xbox as a client.
However I think Fuppes will do it. http://sourceforge.net/projects/fuppes/ but you will probably need to compile from source as I think packages are scarce.
There is also twonkymedia server but this is non-free (in both senses of the word)