I spent alot of time trying to stream files to the 360. I ended up giving up on it because The fast forwarding would only work right with a WMV. I would get a network congestion box on the screen no matter if it was wired or not and anything but WMV while trying to fast forward. Even with .MPG's which is a supported file.
The other thing is the 360 is too loud when your trying to watch a movie.
To top it off MS(
) allows some files to be used in the regular dash and totally different ones used through the media center dash.
I'm waiting till it gets hacked. I hope it works out for you.
I'm with you on the loud part. It's okay when you're playing games as they are almost always at a constant volume but with the dynamic range of movie soundtracks it's easy to get distracted by the noise. I don't like having a PC on all the time to act as a server either. I've got a nice little 1TB NAS box that houses all my movies and music and it works like a dream with XBMC but will not work with the 360. My fingers are crossed for XBMC on the 360 or SMB support.
I'm keeping my eye on a the Appletv and the popcorn hour.
http://www.popcornhour.com/onlinestore/This streamer is having a good start so far. For all the formats it can stream it only cost $180.00. Right now they are back ordered for a while, but I'm keeping an eye on them.