Getting a sound card for Logitech X540

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.