From my understanding its not that it doesn't sound as good, (unless the onboard is like 5.1 and the sound card is 7.1) it that when you play games and stuff it lowers your fps because the CPU has to work harder.
All wrong. Soundcards haven't had any FPS impact since the first C2D's came out. Nevermind audio isn't H/W accelerated anymore.
The primary advantage of a soundcard is vastly superior sound output compared to onboard. Period.