Wrong. Onboard sound quality has improved significantly in the last several years and onboard sound is now more than adaquate for your average PC speakers, cheap sound cards are worse than the onboard sound on higher level boards. If you only have a $40 speaker set then a sound card isn't going to help, if you are dropping $150 on a nice surround sound set then a sound card is justified, but for your average user onboard sound is much higher quality than their speakers are capable of reproducing anyway so a sound card would be a waste.
The integrated on my ASUS M4A79T Deluxe is about the same level as the Soundblaster Audigy SE that i was using in the past, cheap sound cards aren't going to give you a big boost in quality over a good integrated chip.
im using roccat kave headphones aswellas some £60 creative speakers, i can hear a massive difference between my onboard and my xonar dg, the sound quality is much better and the interference i could hear through my headphones is gone