I'm going to go out on a limb here and say that all PC audio is exactly the same at the line out jack, be it a card or a mobo, no matter how much you spend. To all intents and purposes. Unless you're a recording studio. The difference between good and bad speakers or headphones is infinitely greater than any slight difference between PC audio arrangements, which all produce sound adequately linearly across the range of human hearing at very low distortion levels. The engineers have that pretty much dicked in the new millennium.
If you want to throw money at your sound (I know I do), put it into better headphones or speakers, not that the ones you have are bad. How do you like the Sennheiser PC151s? I'm looking for headphones right now.