odds are the 20 ones use the same or inferior sound chips as the one on your motherbd. You are better off investing in better computer speakers than a better sound card until you are ready to spend sever hundred dollars on speakers alone.
what I'm trying to get across is that there no point in buying a xonar if you have $20 computer speakers... you get the xonar when you are ready to buy speakers that wont make you cringe. I have 100w computer speakers that sound really good and still use my onboard sound because my hearing isnt all that great and I cant tell the difference in sound from $100 speakers vs $1000 ones (too many concerts in my youth)
well yeah, expensive ones are really for audiophiles, no use buying them if you cant recognize the difference (im not an audiophile).
i have a xonar that was i think $60 if i remember it right, and a decent 5.1 speaker.
There is one other cause to buy a cheap sound card. I have serveral friends that I play Battlefield Bad Company 2 with who get crashes in that game when using realtek onboard audio, which is what ~98% of all onboard audio is. Im sure its not the only game that has that issue from time to time.