I've been playing around trying to find a decent media extender for my NAS array and all the dedicated boxes I've tried are complete pants, especially for the price.
I'd heard about the xbmc and was about to delve into the mod-chip world for xbox when I discovered someone was already doing it on ebay pre-modded with xbmc installed for less then £60.
Using it has been fantastic, it plays everything I need and I can play games on it too. If you stick a larger hdd in it use can dispense with the NAs as well if you haven't got too much data (my NAS is 4 Tb so not an option for me).
Do yourself a favour, until these boxes get a decent o/s you can go wrong with an xbox :)