If you have some more money to spend, check out the Creative X-Fi series. Avoid the Extreme Audio, but all the others will make a big improvement over your onboard sound card.
Thanks for the answers, i already bought a nice soundcard, which is now connected with my Onkyo.
Now, i am fighting with some preferences, because i can only hear the testing sound until now, but no sound from any other program until now...it works, but only in the testing options.
Edit: I thought, i cannot add another card, but then the dealer meant, i should have a short PCIE slot. And he sold me a card for it. And yes, there is a short PCIE slot on my mainboard.
And now i have a optical+digital connection from my PC... for me, as an optometrist, a must. Hehe.