No, it's not worth it in most situations, the quality of sound is usually bottlenecked by the player(headphones, speakers etc) and not the sound card. Unless you have a $300 5.1 surround speakers or something along that line, and you have professional demands, you may consider a new sound card.
$300?...I play music on my Martin Logan Vantage Pair from my Asus P6T onboard sound it and sounds excellent.
If you want surround sound effects and other stuff, perhaps a SC will help, but for music, I can assure you, the sound quality will not differ by much, if at all to a noticeable level. And I have tried playing them from a friends soundcard, not a difference, but all this testing was in stereo for music, which is what I use my speakers for, not movies or gaming effects.
So really it depends, but surround effects and other stuff, I am really not sure, but for stereo music playing, it will not do anything, unless perhaps your onboard absolutely sucks, but I told you the results of my comparison, up to you.