Sound card vs motherboard sound?

No, almost all sound cards come on-board on the motherboard and they are very good.

I also doubt that you will notice a difference. Sound cards are from a day when all motherboards had terrible sound and a lack of channels...that and single core cpus. So any computing chore that could be removed from the cpu's "to do" list was beneficial to overall speed. Today, the sound is pretty good on most motherboards and multiple core cpus easily handle multi channel sound chores, thus sound cards aren't that great of an upgrade.

That said, most sound cards do sound better than onboard audio...some out of shear quality, and others due to the fact that adding sound via a soundcard reduces a lot of the motherboards circuitry "noise" from being picked up since the card is literally/physically further away from the motherboards electronics. Thing is, 90% of the people on the planet wont be able to tell.

So unless you are the audio aficionado with $1200 speakers...you probably wont tell at the end of the day. It cracks me up when people buy the most expensive sound card ever to make their MP3s sound soooo good. (thats an audio engineer joke, MP3s all sound like crap regardless of what you play them on, poo in/poo out)

Just to further complicate things, it has been my experience on some motherboards, when overclocking, the onboard sound will start to degrade...and this makes sense, more voltage flying through the board more circuitry "noise" is present. But this is different on every board.