It really depends. The best bang for your buck sound card right now is the Sound Blaster X-fi XtremeMusic, but you wont see much difference if you have very cheap speakers. with a good sound card, it lets you enable eax, hardware audio and all the fancy audio features in games. you cant do that if you have a cheap sound card or onboard sound. If you already have decent speakers, get the sound card, but if you have crappy speakers, get speakers, and then later a sound card.
so, umm, err yeah