It's all in the SNR nowadays, unless you are concerned with gaming. ALC888 is better than ALC892, which is just an "adequate" chip. A lot also depends on the motherboard's implementation and noise protection circuitry so you cannot just compare two chips like that.
For example, the ALC892 on my Biostar motherboard sounds much better than the ALC892 on an Asus motherboard I have lying around.
In general, VIA's audio chips are very, very good. Onboard audio, what you should look out for is detailed below:
- Realtek: ALC887, ALC889, ALC898
- VIA: VT18xx, VT17xx, VT2xxx (all are good)
As for your particular question, if you have a decent pair of headphones/earphones/speakers, the Xonar DG will make a difference. It may not be significant, however.