Best advise I can give is to purchase a Sound Blaster Audigy 2 Sound Card (Platnium version) and run a very long fiber or coax digital cable to your DAC. The Audigy 2 has high end DACS in it, and is a little expensive, so you could get the Extigy which is a little cheaper (or the Audigy v.1) and still use digital cables to make the connection. Hercules also makes a good sound card that allows a digital connection.
http://us.hercules.com/products/showpage.php?p=57&b=0&f=1
http://www.americas.creative.com/products/category.asp?category=461&maincategory=461
http://us.hercules.com/products/showpage.php?p=57&b=0&f=1
http://www.americas.creative.com/products/category.asp?category=461&maincategory=461