Depends on the speed of the dvd drive, for example the ps2 dvd drive, at the very edge will be getting around 55Mbps.
With 100Mbps ethernet, if you can sustain even 80Mbps, that's a great improvement over the speed the dvd drive offers.
Xbox drive is 2x-5x (Depending on location on cd most likely)
So 100Mbps would be more than sufficent for streaming.