Ok first of all what kind of setup are you using?
For my setup I have a Panasonic head unit that has Auxillary Inputs that allows me to run the sound from my computer through the head unit as it distributes the sound evenly through my speakers, this way it also amplifies the sound too.
I too also am going to put in an audigy card into my car, and am wondering about how to make it 5.1
Im not sure if this is possible by running the sound through the headunit because it only has 2 rca plugs and theres over 6 speakers in the car. So this give us a bit of a problem. How ever to just use the speakers off the sound card you can get an adapter that converts your plug from 3.5 to RCA.
In saying this there might not be alot of point in getting a audigy. But I guess you could try anyway.