I am quite happy with my Xonar DG, if you want more info on this though I check out Head-Fi and post there they have a far larger knowledge base when it comes to audio. As thats what the forum is filled with nothing but audiofils. They are the ones who recommended I pair up a Xonar DG with Superlux 668b and I am quite happy with that setup.
For for me I heard a bit of a difference in sound going from the integrated sound card on my mobo to the Xonar DG, not night and day but it still there. I can probably spot it better now as I'm more accustomed to higher quality sound now since I have gotten used to my setup and I was using some bad sounding headphones piror to the upgrade. For me though it was well worth the $20 upgrade, check out amazon/newegg/tigerdirect/etc usually one of them has a $10 rebate going on so you can pick it up for around that price.