If you're getting a $400 headphone, I highly recommend you also get the equipment that'll make it sound like it should. That means definitely getting an amp. If you're gaming from your PC, a soundcard is always a good idea. The one you mentioned gets solid reviews around here. However, if you're getting an amp as well, you don't need to spend much on a sound card as you'll just be using it for DSP and DAC. I'd recommend a Sound Blaster Z or Omni (whichever is cheaper when you decide to buy).
Which amp should I get? Im new to the scene. Whats dsp and dac?