I currently use a rebranded version of this for my HD518:
http://www.newegg.com/Product/Product.aspx?Item=N82E16829128010
Sounds great without an amp, and already really loud at 30% volume.
I also used to use the Realtek ALC888 and ALC892 for PC speakers, but the HD428 I bought 2 years ago sounded so anemic when plugged into them. I got a Xonar DG and it was a huge improvement, at least for me. My music sounded like, well, music.
Sadly, I couldn't use the Xonar DG anymore because I switched to a mITX motherboard so I just the external DAC above.
DG - PCI slot
DGX - PCI-E slot
Personally, I'd get the DGX because a lot of new motherboards today do not have any PCI slots anymore. It would be a bummer if you got the DG and the new motherboard you wanted didn't a slot for it.