Using a Mac, I don't know if a PC soundcard messes up digital stereo sound. It would surprise me if it did anyway.
What's important is this:
USB is limited to 'CD quality': 16bits, 48kHz.
S/PDIF, coaxial or optical, allows 24bits, 96kHz signals.
So, if you intend to use FLAC files, USB is not a good option.
(There's some DACs that allows 24bits over USB, and some that only allows 16bits over optical -- but the Caiman is not one of those.)
USB and coaxial uses metal cables that might introduce electrical noise from your computer into your sound system.
Optical cables effectively insulate your hi-fi from electrical noise.
Thus, an optical cable is the best choice for connecting a computer to a DAC.
If you'll content with 'CD quality' sound, you might consider a wireless connection, like Apple's AirPort Express or Apple TV, or the slightly more expensive (and in my opinion less versatile) Logitech Sqeezebox. Apple's equipment don't have a coaxial output though; only optical.
And:
There's no way known to science digital cables can affect sound quality -- it either works, or it don't. Some people hear subtle differences with different cables -- but then some people also see elves and fairies...