For high-quality DAC from computer audio, check out www.stereo-link.com. They make an external USB DAC that avoids the electrically noisy environment inside the computer case and provides analog output much superior to most PC soundcards.
Those with Macs probably don't need the stereo-link: Macs have long had excellent built-in audio capabilities with impressive DAC. If you have a Mac, check out the excellent discussion of digital audio issues at http://www.macintouch.com/usbaudioreader.html before spending additional money on either an internal audio card or an external DAC such as the stereo-link.
