You'd install OS X from the install disc/flash drive.
But why would you spend money on a part that apple is going to give you for free? At least get the free drive first and use it elsewhere.
So, the shortcoming with coax/spdif DACs relies on the fact that they recover the clock from the digital signal and most DACs then use that clock. That means that an external DAC with a good clock will typically use the poor quality clock from the integrated sound card, negating some of the...
For unrelated reasons, I would not use a spdif dac like such on your computer without buying a better digital audio output.
If your speakers/sub will play nice with being switched at the AC socket, you could (ideally) plug them into a timed power control unit that would switch them on and off...
....If you want proper sound/surround sound. Its sort of a corner case that can sometimes give out all kinds of headaches in windows vista and up.
290x -> AVR is the cheapest and should be the most compatible excepting old games.
Many features of your sound card are going to be rendered...
What is best/cheapest for you depends on what you want to do.
For SPDIF:
If its stereo output to your receiver exclusively, then the only difference in SPDIF sound cards is how good the clock signals are. The ZxR is likely overkill and unnecessary.
If its surround sound over one spdif cable...