Well, I would try and go for a better sound card, like the turtle beach santa cruz or the Sound Blaster Audigy, but the speakers are awesome. I use the speakers with a SB live and it sounds good, though everyone I talk to seems to reccomend getting a different sound card.
Yes, it will output 4.1 On the back of the SB live 5.1 card there are ports for your front and rear speakers. the only difference (I believe) between the SB live and the SB live 5.1 are that the regular live has a digital output while the 5.1 has another speaker output (or sub) in it's place. no, the z-560's are not digital.
Let me know what ya find our if you buy them, I am getting a pair of the Creative Labs DTT2500 speakers and was wondering how they compared to these.
From what I hear, there really is not comparison as the Z-560's are 400 watts and pretty much blow out any other PC speakers except for the Klipsch setups (alot of reviews say they are both the same, I have even read 2 that said the Z-560's sounded better) and for $150, you can't beat the logitechs.
Hope that helps
-Spike