It took me few tests and in the end I decided to go for XBMC running on Windows… Why? Cause are too many good things from this combination beside the troublesome part of setting up the WiiMote to work with it.
Right now the best solution I found so far is to run Win XP Embedded with SP2 and XBMC from a flash (less then 500 Mb including XBMC, BBerry drivers, and other applications/drivers). I am using a NAS for storage (NAS900) and share through SAMBA the videos, music and pictures. I have tried several options for remote control and the best so far is… iPod Touch (or iPhone) and Xmote. This will allow to control almost everything but require TV for interaction. Another option (but I’m not really happy with it) is to use the XBMC Media Centre remote. This will allow interaction with your box without need of a TV, but with several limitations also… :-)
Using Ubuntu as OS have advantages and disadvantages. But in my case using Win XPe seems to have more advantages: stable, more skins, better media handling and video drivers, stable web sever for a better web integration (e.g.: XBMC running on Ubuntu amd64 throw errors when trying to access the web remote option, on x86 version is also pretty unstable also, CPU often goes to 100% when navigating through the interface and on some configurations even during playback), up to 1920 x 1080 @ steady 24 fps with less then 50% CPU load on my Sempron 2100+, terminal service for remoteaccess and best of all, at least for me, is the Video Plugin option which is a nice addition to the system.
What will be next? Make it start faster, “hide” any trace of Windows in the UI, make it more to look like an appliance ;-)