You can just plug the HD-audio connector to your motherboard. This will give you the onboard card running when you plug in headphones. This should have the same quality you have with the onboard card right now.
If you do get a sound card, check the specs to make sure it has a front port audio or HD audio connector on it. Some older cards did not have that connector leaving the front port audio useless. As long as the sound card has that plug, you can plug that cable into the new sound card and it should work fine and at good quality.