In most cases you do not need a sound card. It is just needed for audio enthusiasts who want pure gaming audio performance / crystal clear fully detailed studio level music, etc.
Another advantage is minor frame rate increase, but that is all. Motherboard's internal sound card is more than enough for everyone's needs.
However if you still want to buy a SC, go for the cheapest you can find from one of the good brands. In this case Xonar DG.