DAC can upsample higher than most sound cards, e.g. 24/192. With an budget of 80, there isn't a lot of overwhelming headphones in that region. If you can find the Sennhieser HD518s, AKG 240/242 etc or up your budget slightly and go for the almighty Beyerdynamic DT770, which are in my humble opinion one of the best set on cans on the market in the budget range.
The Xonar Essence STX does 24bit/196khz ( if we are talking about the same thing, im a noob so dont blame me lol )
For headphones im probably getting the Beyerdynamic DT770 Pro's which yeah ive heard awesome reviews,
Just wondering if an entry level DAC is better than a pretty awesome soundcard ( such as the one mentioned above )