Well, you can use the 5.1 speaker setup right away, but if you want 5.1 sound, you'll need to pick up a sound card. If you're looking mostly for music sound, you'll want something with the VIA Envy 24HT chip on it. Depending on which card you look at they can run from $25 up past $100. Gainward and Chaintech have some cards on the cheap end. On the more expensive side you'll be looking at M-Audio or Audiotrack. It depends on what you want in a sound card.
If you want to be gaming, you might look into Creative's Audigy 2 series of cards.