Question about S/PDIF versus USB

[linuxfan]

Hello.
I want to build a Linux-based music server using a small, fanless, low-power x86 computer.
And I want true high-fidelity audio playback. Obviously I will be using uncompressed audio files, or lossless compression.

Thanks for the valuable information on this forum. I have spent quite a few hours reading many threads, and it appears that my project is viable.

I am now contemplating the various external DAC options, and there's a basic question I need to ask:
It appears to me that there are 2 basic choices when it comes to connecting a DAC to a computer: S/PDIF output of a conventional PCI sound card, and direct USB connection. Could someone please explain the difference, and relative merits?

Regarding USB connections, I'm guessing that the DAC can access the audio data stream directly from the computer's USB bus?
In Linux, this would probably require the generic snd-usb-audio (ALSA) driver?

Regarding S/PDIF, I wonder whether a conventional sound card is somewhat wasteful of resources, since all I need is the S/PDIF output? If I'm right, is there such a thing as a PCI card which is ONLY a S/PDIF adaptor, without any onboard D/A conversion?