I have no current problems with the onboard audio and it sounds fine to my ears, however I have been told a good sound card will sound like night and day.
That depends on whether you're upgrading from a bad soundcard. I remember some onboard audio solutions in the late 90s and early 00s that had DACs so terrible you could hear the "essss" on raspy female vocals being distorted by some double-digit percentage, in which case there was no question that the sound quality was junk. Now, these sorts of problems have generally disappeared, and the biggest upgrade advantage is that component soundcards can have better grounding and shielding, which may improve the signal-to-noise ratio.
Rather many other claims made about hair-splitting quality differences in audio arise for the same reason that most people think an $80 bottle of wine tastes amazing: you just spent $80 on something ephemeral, so it had sure better. IMO what this means for your soundcard choice is that if you need an expensive hobby, buy the pricier unit and tinker away. If you don't, go with the less-pricey card. It will be better than your onboard solution in the ways that definitely matter, and meanwhile there are a lot of other ways to spend that $100.