They probably have about the same capabilities, but the PCI card will always sound cleaner. Onboard sound in my experience always suffers from noise. It's tough for it to be isolated with so many high frequency components packed on to the same board.
Case in point, I just built a PC for a friend, and he decided to stick with the onboard AC97. I used headphones during the setup and once I got the sound installed, there was a hum everytime the mouse was moved. I'm assuming it was because the USB header was too close to the sound chip (USB mouse).
Yes there will be a difference in sound quality but not all people can notice this differeces , its all depend on your ear and what kind of speaker you are going to use...for headphone i think with on board you will notice a humming sound...
You can always try the sb live first ... if you dont deel the diff then just go with the onboard