To get totem to play dvds in gnome - you need to yum install totem-xine (though you may need to install libdvdcss2 as well).
To get mplayer going on FC4 - do not
use the yum packages. Instead, source install following the instructions in www.mjmwired.net
BTW: getting mplayer going is a very common question in these forums - just do a search and you come up with me giving this advise over and over... and over...
rpm dependencies are automatically sorted via yum, the RH equiv of apt-get. You can also use the yum extender (RH eqiv to synaptic) for a gui if you prefer (yum install yumex).
GXine and xineui and so on are good, but the totem xine interface is great.
MPlayer is probably overkill if you just want to wach commercial dvd's. MPlayer seems more concerned with creating and editing dvds and catching streaming media and so forth.
Debian based distros can install totem-xine (apt-get install totem-xine) but must also install libdvdcss2 (apt-get install libdvdcss2). mplayer can be installed this way too - apparently it works - but you still get more functionality from the source install.
Note mplayer-hq developers reccommend the source install.