well this is a loaded question.
The onboard sound card (that comes with most motherboards) generally if you look strictly at the specs are better quality than the human ear can hear. Even the ones on the low quality motherboards generally "in theory" are better quality then the human ear can differentiate.
THAT SAID - if you're a bit of an audiophile, you might notice some differences between the audio quality of the sound on motherboards. Many boards lack high quality gold plated interface jacks, many aren't EM shielded, meaning all the electronic devices in your motherboard will influence the quality of the audio as well. This is why I said the question was a bit loaded. In theory, going strictly by the specs for each of the onboard sound cards on these boards, they all should be far better then your ears could ever differentiate, yet in practice you probably will be able to tell.
High end motherboards don't necessarily mean their sound is any better too, unless the sound card is electronically isolated, many of them also have the same issues. Even on the ones who DO isolate the sound card, many lack quality audio jacks on the IO panel. Finally, if you have some crappy dollar store speakers you'll never tell the difference no matter what motherboard you have anyway.
SO - assuming you have high end speakers
AND - assuming you have a motherboard which isolates the sound board from the rest of the parts, plus has gold plated jacks
then you don't need anything else.
If you have high end speakers but a "normal" motherboard, you probably will notice a lack of audio fidelity, and getting a well made sound card could be an option
If your speakers are junk, I wouldn't bother with a high end sound card.