I would agree with XV that a soundcard is nearly always a better option as they are more controlable - the software is better & give much higher fidelity. It comes down to the value you place on sound quality really.
Beware though, adding any product from Creative into a Vista system. There are driver issues with them, particularly on the Audigy & Xfi range, which while surmountable, have had creative users baying at Creative on the user forum.