The good old Xbox (not the 360 one) serves as a pretty damn good media center in my opinion. I use the Xbox Media Center. It has also web interface for controlling the playback of music/video from a web browser. I've used it with my N800 and it works relatively ok. It isn't optimized for this small screen, but I'd imagine it would be easy to modify it a bit to suit the screen of N800 a bit better.

Good about the Xbox is that it is now really cheap. Of course you need to have a modchip installed, but those cost only something like 20$.