I thought a soundcard will always be better than onboard. My tech friend told me that instead of the cpu processing all the signals, it will now go to the soundcard and straight to your audio device (so you free up some cpu usage there). I myself was looking into a soundcard, particularly the Asus Xonar DX ($90), and then later buying some headphones. Right now I have a 2.1 setup (altec lansing) hooked up the onboard and I must say it is not that bad... so I don't know if I really need one. I'm not sure if spending another 90 will improve anything really, as everything seems fine. But the fact that I CAN and USE a soundcard makes me want to just get one...
Make sure you give us an update after installing everything. I want to know if you notice any difference.
Edit: Can anyone tell me whether HT Omega Striker or Xonar DX is better?