Yep, what he said..
But yeah, any music you have on the computer that is played through that card will sound pretty darn good. You can also use it as a DAC if you want, by hooking a CD player into it's optical input and using it's analog output (with that cable I told you about). When you do that, however, you have to switch the card's "clock" from internal to external and source it off of the digital input.
But, you'll probably not appreciate any improvement unless you're feeding it into a decent set of audio equipment - two lil' ole plastic computer speakers won't do it the justice it deserves.