It's all depend on how much are you willing to pay for a Sound Card. Any sound card is better than the onboard. But if you want to pay more than what you pay for your speaker then Creative has an edge. Their x-fi line-up.
Still your speaker is limited to the power it has, 40 watts total means 23 w sub and 8 w on satellite so if you reach the speakers or amp limit then not much you can do. Having a good soundcard gives you better control of sound quality the speaker. You may also be suprise that your speaker produce better sound than when it has no add on card.