Is your on board sound working or not?, if it is just giving poor quality output to your stereo system then by all means go for a new sound card (there are plenty of cheap ones to choose from here
http://www.newegg.com/ProductSort/Ca...p?Category=36). If it is not working then that could be indicative that the motherboard is failling, I keep a cheap but reliable soundcard in stock for just that reason, you don't want to go fitting a good sound card to a bad motherboard just in case, if the motherboard throws a wobbly you are never quite sure what she will take with her.