I'm going to agree with some of the previous comments and suggest you get the headphones you want and worry about adding a dac/amp later if you feel the need.
Also to clarify, you wouldn't need to buy the soundcard, dac, and amp. The soudcard serves the same function as an external dac. I think the recommendation above for the ASUS xonar soundcard is a good option. If at a later time you want to feed the headphones more power you can always add an amp later.