DWR and 4est are both right.
No sound card especially on a Mac. If you insist take a look at the pro audio offerings from Apogee and the like
If you already have a DAC you like, what you will probably need is a USB to SPDIF box - Blue Circle and Hagerman both make them (among others) - so you will come out of the Mac via USB, go to the box, then come from the box to the DAC via SPDIF or Toslink.
FWIW if you get the right Toslink cable (ie Wireworld SuperNova 5) G5 to DAC is great - I ran one into a TriVista for a few years, no complaints.
If you need a DAC, then a USB DAC is the way to go. Take a look at Wavelength Audio, Apogee, Paradisea and too many others to mention. The best units are the ones that go USB into I2S with no SPDIF interim stage.
All very easy and the results are great. Just make sure you check "Error Correction On" in the iTunes preferences.