Sound card or onboard?

If you have both, try each out and pick the best.

If you are looking to buy a sound card, try the on-board sound first. Most of the time you will find it to be excellent.

Only if you have high end speakers and a good ear would a sound card be worthwhile.

In past history, a sound card saved some cpu cycles, but with today's faster cpu's and multi cores any cpu savings is a non-issue.