For a reasonably new PC, the onboard sound is usually pretty decent... What exactly are your requirements? There are many options such as the Creative SB Live 7.1 series to the Creative Audigy... Most Creative Soundcards are pretty good and there is option for various price ranges... What price range are you looking for?
Also, what speaker system are you planning... Sometimes, certain speakers and soundcards complement each other better than other combinations...
Arun