Sounds like a waste of time if that was your sole reason to reflash. There are some extra codec packages that you could install, For the rest, I believe the best result for video playback on the n900 is achieved by "mplayer" for the command line. It allows the best fine tuning and the most options of all the programs available. Code: sudo gainroot apt-get update apt-get install mplayer /* Only installable, of course, when the repo's are back up, or you'd need to use one of the mirrors. */
sudo gainroot apt-get update apt-get install mplayer