Sound is subjective. What 1 person thinks sounds great, the next will think sounds like total crap. Like all the boom boom booming speakers I hear in cars these days, I have no idea why anyone would want to set and listen that crap. Since you have the card, it seems irrelevant to ask peoples opinion, you can try it both ways yourself and see what YOU think sounds better.
My 2 cents:
Never buy a sound card if your board has sound, try the onboard first. If it doesn't do a good enough job for you, then start shopping for a sound card. Most people will do just fine with onboard sound, but not everyone.