kmplayer is only a front-end/GUI for already installed player like gstreamer and mplayer. BUT: for gstreamer the mediaplayer-gui is the best. for mplayer i would say that SIB is the smartest GUI - it's not as shiny as kmplayer but it's way less resource hungry and it seems that it starts mplayer with the best options to optimize it for use on the n900.
mplayer + SIB simply kicks ass. forget gstreamer and vlc for now. ;)