Question....what kind of system are you connecting your Mac to?
1. Mac OSX uses "Core Audio", which means you don't need a "soundcard" for most applications. "Core Audio" handles audio files extremely well, and this is one reason why Macs are used for professional audio applications. Sound cards can be purchased for Mac Pros, but it's normally for those professional applications that require studio mixing and mastering. Most people that have correctly setup a Mac for music will tell you that they are an excellent music source, just using the standard digital outputs (optical, USB, FireWire).
2. A DAC's main duty is to take those digital outputs and create a good analog output. In the case of music, the DAC should reveal the space and texture of the music, the kind of thing that gives you goosebumps. DACs sound different because they are "tuned" to have a character. I view DACs the same way as phono stages and cartridges. They sound different because the designer is creating a character or style.
3. Some computers don't have standard digital audio outputs (SPDIF coaxial and toslink) so if you want to connect one of those computers to a DAC that only has those inputs or a surround sound processor/receiver, then a USB-to-SPDIF converter can be used. In the case of your Mac, the simplist (and best way IMHO) is just use the optical output into your DAC or processor (if your DAC/processor has an optical input).
4. If your DAC/processor doesn't have an optical input, yet has a coaxial, then I recommend the HagUSB by Hagerman Technologies. It will convert USB to coaxial very well, and is USB powered, so no "wall wart" power supply is needed.