Onboard sound is known for causing signal noise, Mobo's are too "hot" electronically to successfully produce a clean signal. If you can detect this noise, you should be considering a Sound Blaster for gaming. If you are into audio editing, a Turtle Beach card may be more in your league.
I have a SoundBlaster X-FI and it is incredible with my headphones. Before i got the X-FI i had a SoundBlaster Live! 5.1 and i could tell a huge diffrent from the two. I recommended the X-Fi all the way, thats just my opinion.