Doing 5.1 or better tends to take up system processor I think(I'm not 100%)
Dolby Pro logic II is nice in that it provides a 5.1 experience(better than stereo) but it doesn't come out as good as 5.1 source material however it doesn't use much/any of the systems processor to provide it which on the wii is actually a pretty good idea.
http://www.dolby.com/consumer/technology/prologic_II.html







