Any money spent on a sound card is better off spent on speakers and a good DAC, which often come together.
High end sound systems and speaker systems these days have digital inputs, thus an onboard DAC. If you're using a digital output on your motherboard to connect to a digital input on the speaker, the onboard sound card has ZERO effect on the quality of the audio. The bits are traveling directly, unmolested from the application generating them to the amplifiers in the speakers.
Now, if you have audiophile-type equipment that uses analog inputs, then YES, the analog sound you feed into those inputs needs to come from a high quality DAC. High end sound cards tend to have good DACs, but you can get the same effect by using an outboard DAC, which has a digital input and analog outputs, and is also AWAY from your PC, so your analog audio is less likely to be affected by interference from the motherboard or power supply.
You can get DACs with USB inputs, but USB adds latency so is best avoided for gaming. For music, go to town with a USB DAC; it won't matter there.
The gist of it is, the most important component is the DAC. The DAC completely determines the quality. Everything else is just hype.