Adding a soundcard is always a good investment especially if you're an overclocker. Overclocking can mess with the bus frequencies which in turn can mess with the onboard audio chipsets sound output. A lot of times under a decent overclock you might notice hissing, popping, or other interference while using the onboard sound.
This can easily be eliminated by purchasing a soundcard. You will also (depending on the quality of card you purchase) get a much better sound reproduction and overall sound quality from using a sound card. I definitely suggest getting one if you're a clocker.
Cheers! :beerchug: