If MP3s is all you anticipate listening to in this manner, then it might not be worth your trouble to install a sound card just for the digital output.
As long as you have sound capibilities on the computer then you could just run the 1/8" stereo out you normally use for PC speakers to a stereo Y splitter
like this that separates the left and right signals and gives you
female RCA plugs. (I was thinking more of a cable version rather than the rigid one I linked you to...but you get the idea.) Then pop a regular RCA cable from the adapter into an available input on your receiver.
Like Seth said, you're not going to get CD quality anyway. You're not likely to be able to tell the difference this way vs. going to the trouble and expense of a new sound card and a coax or optical cable. This way you're looking at about $10 max...less than $5 if you already have a set of RCA cables.
Your computer isn't exactly a monster, but it's fine for this. Playing music doesn't require much horsepower.
