Who can say after reading Vader's comment. Haven't compared my recently purchased Xonar DG to on-board audio, although I'm certain it's easily better. I have compared my older X-Fi Xtreme Music to various on-board audio solutions a few times. The X-Fi is far and away better. The D2 is certainly a better card than the X-Fi, so the difference should be pretty apparent. Everyone is different though, and some can appreciate or hear more detail than others.
It's impossible to say for certain if someone will notice a significant difference between on-board audio and a sound card. Normally, one would say yes, but that's the people who find a sound card to be far superior. There are a few people who have proven that on-board is more than enough, by noticing little to no difference compared to a sound card. Until someone has used a sound card, and compared, they can't really know for sure.
I should say, quality of speaker and headphone will play a part. Although, I have seen people say they can tell the difference with cheaper speakers. I certainly noticed a difference between my X-Fi and on-board audio, using Creative T6100 5.1 speakers, a few years back, and they were crap when it came to music.