The correct duplex setting is entirely dependent on what device the other end of the ethernet cable is connected to. If it's a hub, half duplex, if it's a switch or another device that's not a hub, full duplex. Just leave it set to automatic.
In fact, I don't recommend that you change any of the Ethernet settings unless your ISP instructs you to.
(Note that reducing MTU is usually only necessary for DSL using PPPoE -- it typically likes an MTU of 1492. If you don't use PPPoE, the default of 1500 is usually correct.)
tooki