Would a sound card improve optical audio to my receiver?

Probably not. Optical carries digital audio to the receiver, and the receiver converts it to analog for playback. The sound card's DAC isn't in that signal path.

Stereo PCM is still digital audio, even though it isn't a Dolby surround bitstream. If the onboard optical output and sound card send the same unaltered data, there's no reason to expect a sound quality upgrade from the card.

A card could be useful if you need a particular processing feature or your current optical output has problems. Software effects, volume processing, or format settings can change what gets sent to the receiver. That's different from the card's DAC making optical audio clearer, though.

Save the sound card money for speakers or headphones if you're looking for a noticeable upgrade. Your receiver is already doing the digital to analog conversion, so adding another optical output doesn't address that part of the chain.

An external USB DAC is another option for stereo, but that's a different setup. You'd connect its analog output to an analog input on the receiver, so the external DAC would handle the conversion instead. Whether it sounds better depends on the equipment and settings, not just whether the DAC is external.