Without knowing your specific motherboard, most motherboards have sound "decoding" already built in. That is how you are listening to your sounds right now without the sound card. Probably realtek audio drivers. I was looking to get a soundcard and I did alot of research on it.
I recently built a computer that i play all my games and movies on that actually is plugged into my plasma and home theater speaker system (ZVOX 580). Mostly you get sound cards to either take advantage of digital outputs for higher bitrates on exceptional speakers or better headphone amplification. You will notice a significant difference with really good headphones with the Xonar. If you were using just ok computer speakers, then a sound card is not worth the money.
You haven't said what you are primarily using the computer for, but I am assuming games. Xonar is good all around but I have heard it makes a big difference with good headphones. If you just have cheap headphones, don't waste the money on the card. Just get the headphones (you should have atleast 75 dollar ones). Look for good rankings on them.
Also once you get the card, make sure if you use itunes to buy higher quality mp3s. If spotify you may want to upgrade to premium. GL