many people like to use the dragonfly usb dac as it puts out very superior audio quality plus it is small and mobile and can be quickly plugged into any computer. a dedicated sound card is more for home theater stuff. what headphones are you looking at? i can tell you that for sonic clairity a simple pair of audio technica ath-m50's are going to sound better to an expert than the top of the line beats headphones. you can get a dragonfly dac for about $125 and you can get the athm50's for about $125 also.
Thanks, I was thinking about getting ath-m50's or Shure SRH 440's.