The first thing I'd consider is your existing speakers. If they're crappy, then onboard sound is fine. The standards of onboard sound have improved a lot.
But if you want a real audio experience to go with your games, then a good sound card/speaker combination is really worth it. Although to some extent it does depend on the genre of games you play. With a FPS I think a good sound setup really adds a lot.
Wingding - proof of the need for genetic screening