You cant play videos in 360p or 480p quality because it would be really laggy. Try using YTube or CuteTubeQML from the repos (both probably still in extras-devel) to watch youtube videos in high quality. For other Flash objects that don't work try Tweak Flash plugin version, it lies to sites that you have Flash 10 (N900 has only Flash 9 and that is causing your non-youtube problems) and makes some Flash games and videos work. It wont however give you Flash 10 functionality.