Hello,
Rob is right on -- these are USB speakers, which means that the 'sound card' is built into the speaker system, and your PC simply transmits a digital signal over the USB cable for conversion in the speakers themselves. You're right -- your existing sound card will simply be taking up space. You can move your sound card to another PC if you wish -- the Z Cinema's internal DAC is plenty good enough that you wouldn't reap much benefit from upgrading it.
And no, as with an ordinary media center remote, the Z Cinema's remote does not control the mouse pointer.
Cheers!