well for stereo sources, my sblive value will just send everything to both front and back speakers
i'm not sure what else it would do other than that...
the other advantage the 5.1 has over the older lives is that if you have a speaker system/receiver that takes 5.1 inputs, the 5.1 can put things into all the outputs
and a 4.1 card will only give you choices of outputting tostereo and have simulated prologic, or outputting front left/right and rear left/right channels (with no center and no subwoofer)