I know it's been discussed here before, but I have yet to find substantial information to make me decide one way or the other. I have a pair of Audio Technica ATH-AD300 and would like to find which card would be best suited for these headphones.  I am on a budget so saving as much money as possible would be great.  For music, which card would yield the best (noticeable) results?  I know that the DS has an opamp that can be rolled, but is that going to be noticeable enough to warrant paying almost twice as much as the DG?  

Usually it's better to put more cash in to headphones, then a DAC/Amp (or sound card).
So get the Xonar DG, $26.80 with a $10 mail in rebate (Amazon)
and make your next planned purchase for headphones.
Maybe sell off the ATH-AD300 and see how much you could budget for new headphones, to plug into the Xonar DG.