Ok, I've done some research as I've never heard of headphones with DAC's built-in.LOL. You're missing what I'm telling you. A good USB headset doesn't need any of that, it handles all the DAC internally. No internal sound card, no external sound card.
Unless you're wanting a set of headphones that will also work in an iPod or phone or something. Then you would need something (though I would recommend just trying with the Onboard sound first, most motherboards are in the 'just fine' category anyway.)
Seems that they're 'ok', but I think for the price, I'd rather that money go to a higher quality headphone, especially if the encoding quality isn't going to be anything special.
I've found that the ASUS Xonar DG sound card seems to be a really great bang-for-buck internal sound card($30) that has a built-in headphone amp and should go a ways in making a decent set of headphones sound better.
This is just a mockup for now, but it has everything I'd need and its scraping awfully close to $800:
http://pcpartpicker.com/p/1arii