there two issues with onboard vs third party cards. the first is how the sound card sounds to you. having owned both soundblaster cards and using the onboard real sound chip on my asus mb I can tell that the soundblaster live card sounded better. the second issue that was posted a long long time ago online is that on a gaming rig with onboard sound a few clock cycles of the cpu and some of the system resorces had to go when you used onboard sound. with a third party sound card the cpu and gpu did not have to worry about sound in the games.
That was nearly ten years ago at this point. Most people were barely breaking the 1GHz barrier on single core CPUs(AMD 700MHz Slot-A Thunderbird etc..). This is not an issue with todays hardware.