Clearly in an absolute sense there's no way an internal sound card can compete with an external DAC/headphone amp. The Bryston BHA-1, for example, is much better than any sound card and at $1400 or so it should be. Same goes for receivers. But I don't think you can say that an external DAC/headphone amp, or even receiver, is always better than any sound card when you factor in price. It just depends on construction, DACs used etc.
For my current build I bought an Asus Xonar Essence STX (around $175) over an similar HT Omega model, and I don't regret my purchase. A major factor in my decision were reviews from sites like Sterophile
. FYI - the STX direct analog outputs are EMI shielded though that doesn't prevent interference for front panel jack use. When connected to the front panel jack I have occasionally noticed some EMI noise w/ headphone use, but nothing horrible (certainly better than my previous "high-end" creative sound card). As for drivers, personally I've never had an issue with the STX drivers, and the last update was about 2 years ago. Frankly, I haven't had a single issue with the card at all in over two years of use. Lastly, the STX does feature a dedicated headphone amp (power by 4-pin molex) and is rated to drive headphones up to 600 ohm headphones. It has various DSP effects, but what I like the most is the HF or "hi-fi" disables all post processing of the signal when listening to music.
Nothing wrong with using an external DAC/headphone amp over an internal sound card. However, for the $150-$200 price range, viable or perhaps even superior, internal sound card alternative options exist.