I think a sound card helps with performance more than what the majority think. Since I have a low end system I can SEE the difference. I will give you only two examples of what made me make a sound card a must for my system.
My system is very low end, so maybe this is not as noticeable in other systems as mine. But it surely means that the sound card is doing something.
I used to lag a lot playing City of Heroes in a 8 players team. But when I disable my sound, wow I jump +20fps! Same as Vanguard, I get +10-15FPS when I disable sound.
So, I spent many hours playing these games without sound. Because whenever I turn the sound on, lag happens. So, I decided to buy a new sound card. Turned the sound on and wow, no more lag.
Maybe am playing MMORPGs and there are lots of sound involved in these games specially the way CoH is designed. But it sure gave my poor processor some breathing room.
Sound in games reduce your performance, if you buy a sound card you'd improve that wasted processing power. Not only that, you would also get better sound quality too. Better gaming experience.
Get the XtremeGamer Xfi cards. I am getting the XtremeGamer Xfi Fatal1ty Platinum myself