Wing Commander is a bit older than Dolby Digital (5.1). If you're putting together a WC machine, the *only* reason to buy a 5.1 system is to play the Wing Commander movie DVD (which has a 5.1 audio track). You'll need both a 5.1 reciever and a sound card that specifically has a 5.1 output jack (I don't remember what they're called - optical, maybe?).
What various Wing Commander games support is the previous standard, Dolby Pro-Logic ("Surround Sound"). A Pro-Logic recieved takes information from a standard stereo line (so, from your current sound card) and then passes it to several speaker channels (right, left, center and rear, IIRC).
For either setup what you'll need is a reciever and a mess of speakers (plus possibly a newer soundcard for the 5.1 setup). If you go whole hog and get a 5.1 setup, it'll be backwards compatible and you won't need to worry about anything.
I *believe* that every Wing Commander title from Wing Commander III 3DO on supported Dolby Pro-Logic to some extent - including Kilrathi Saga.