I doubt you would even notice the difference in sound quality. Boards these days are coming out with excellent onboard sound.
Dark_Sniper is right, you probably will get better quality sound, but in my opinion, the small sound quality increase isnt worth the money you spend on a sound card.
People will say that using onboard sound, video, etc takes clock cycles and RAM. They are correct, but again the difference is so minimal, its not worth it from that standpoint either, in my opinion.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64