In my experience, HD800 was an acquired taste... My sonical preferences changed throughout my headphone adventure, though my musical tastes remained the same. I really didn't like the HD800 when I first began my quest, while I was in love with my LCD2, but towards the end, I kept reaching for the HD800 (yes, I had the LCD2.2, LCD3, and HD800 all at the same time in the same room on the same table). And then I got to demo the DNA Stratus at my home and it was all magical from there.
However, HD800 wasn't great with any of the lesser amps I used, and it really shows that you need to invest a whole lot more money to squeeze the sound out of these stubborn babies. Don't even get me started on the HD800 with O2.
Don't get a sound card, get a DAC/amp.