a $300-$500 5.1 computer speaker system is not high end audio. The onboard audio on a midrange motherboard will be sufficient for a computer speaker system such as creative's speakers. If you hear hiss, then you can later upgrade your soundcard but $500 is a LOW end audio system no matter how you slice it. If you're attaching your computer to a midrange audio system where the amp itself is in the $300-$500 range and the speakers are real speakers from a real speaker company, then you will want a good soundcard. And do note that if this is for gaming, that in most cases optical audio out will NOT pass 5.1 game audio because game audio is not in DTS or DD format. You'll need a special sound card capable of converting game audio to DTS or DD or you'll want a good sound card with good 5.1 audio outputs and an amp with 5.1 analog inputs (which is getting rare on lower end consumer receivers).
So in summary, if you have a lower end receiver with only optical or HDMI input, then you'll want to look for a soundcard that will convert game audio to DTS or DD and then the analog section of the sound card doesn't matter at all. Or, if you have a receiver with 5.1 analog inputs, then you'll want a good soundcard with good clean analog output and DTS and DD doesn't matter at all. Or if you are going with a creative 5.1 speaker bundle whose speakers sound pretty good at their price point but is NOT high end audio, then the onboard sound from a decent implementation will be perfectly fine. If you're going for headphones over $100, find a soundcard (USB or internal) with a good headphone amp in it.