Renan, stirring up a hornet's nest is always good before we have to sit down to eat. The problem with the configuration that I've specified is that there is absolutely no way to put in any sort of card inside the box.
If what you say is really true, then most recording studios are set up wrong. As far as I know, Firewire is the standard for audio processing in professional studios. Between building my first server in end-2004 and now, I've built several in between - including one that used an audio card. I only tried four cards the Lynx, RME, EMU, and M-Audio, but in all cases, I preferred an external USB DAC. At that time, it was the Stello that we did the DAC shoot-out with at the club last year. With the Weiss Minerva on Firewire being far better than the Stello on USB, I never went back to an audio card.
With internal or external DACs, I always advocate using ASIO.
Willy and John came by to my office today. Willy was concerned about spending the $600 to build the server as he was not sure that it would sound better than just any old PC. I think that he's a convert now.