I've been asked a question which has got me stumped. Apple hardware generally has digital output, so what would be the benefit of installing a high end audio card?
The audio interface you need depends on what you want to do with it.
If all you need to do is output digital audio over S/PDIF to an external device, then the built-in optical S/PDIF port will do the job nicely.
If you need to be able to do high quality analog input and output, a surprising solution is the Lexicon MPX500 FX unit. This unit has been out of production for a while, but is readily available on EBay for $150. You also need an optical to coax S/PDIF converter. If you connect the Lexicon to your mac via S/PDIF, you can bypass the FX unit and just use it as two channel ADC or DAC. As an added bonus, you've also got a high quality FX unit.
If you need more than just two channels (stereo) of input or output, then you should look at a dedicated external audio interface.
In audio-production you need a lot of discreet uncompressed lowdelay in- and outputs.
The S/PDIF audiosignal supports just two uncompressed channels. Multi-channel-audio only works with compressed sound formats and high delays.
You're going to need something to convert the S/PDIF out on your mac to analog audio. Judging by the low quality of the headphone jack on my 13" MacBook Pro (late 2009) anything would be an improvement. The approach you are talking about would work, it depends on what you want to spend. Analog to Digital and Digital to Analog converters can get very expensive. Still, you can get decent (and much better then the analog out on a mac) from a USB interface that is less then $200. Look at the Focusrite Scarlett 2i2 ($149) for example in the low end, or the Apogee Duet in the mid range ($500ish). USB interfaces are the simplest solution. If you are working with a laptop make sure to consider the fact that you are adding one more thing to the bag. BTW This approach skips the S/PDIF out, but is easier to do.