Numbers and statistics indicate that Discrete sound cards offer better sound quality than onboard, period. However, since onboard sound is already pretty good (if you're not using an AC-97 device), the difference is either small or unnoticeable, depending primarily on your ears and your headphones. The biggest bottleneck is really your ears, but it's hard to upgrade those. The next biggest bottleneck is your headphones/speakers. Unless you have rather good pro/studio quality headphones, it's unlikely you'll be able to discern a difference as your headphones would be too shitty in both cases. It's possible that a discrete sound card can make a noticeable difference if you have good headphones/speakers.