Even though you don't plan on using any onboard DACs on your PC's sound card, a good PC sound card with built-in clock synch features and digital I/O will provide excellent high end audio results when you interface your hard drive music source with a high end DAC with word clock synch connections. This will dramatically reduce jitter that can arise from interfacing problems. Lower cost interfaces like the Xitel are similar to other low cost solutions from Edirol and M-Audio which do not take into account jitter generation. If you are serious about PC audio, then consider the RME Audio or Lynx Studio Technologies Model 2 or Model 22 sound cards.
"Sound on Sound", an excellent professional audio magazine from the UK, gave an excellent review of the Lynx Studio 2. The article's author explains why the internal clock provides a deeper sound stage, clearer detail and a more natural sound than sound cards which do not possess a clock mechanism. Keep in mind that the review just evaluates the card's DACs, which goes to show that onboard DACs can sound just as clear and focused as an outboard DAC. Even bypassing the DACs on the card, you can still take advantage of clock synching features. Just go to http://www.soundonsound.com and search for "Lynx" and "RME" to read their reviews. I find that their evaluations and opinions of sound quality coincide with mine. In this magazine, you'll never read a rave review of SoundBlaster sound cards for audio quality.
