This question, always asked, and today the only answer is answered by the question "How good are your speakers?"
Sound cards lost the ability to accelerate anything 10 years ago, so these days getting a new sound card revolves around if you need more functionality (spdif ports) or better dac/amp for sound quality and power.
If you are sticking with a pair of powered pc speakers below $150 then on board should be fine. After that it can largely depend on what type of connection you will use. I have a $300 yamaha receiver with 2 great sony shelf speakers and because the yamaha has some excellent dac converters I make sure the audio arrives to it digitally. Originally I used the HDMI connection but due to moving to a >1080p display I had to switch to its optical line in and for that I needed to buy a xonar card. So if you are looking at a speaker setup that offers a spdif connection, make sure you use it.
If you are going to rely on a good pair of headphones you will probably want to upgrade because the amp in the mobo is probably not that great. There is an affordable xonar card that offers excellent perks for high end headphone users.