There are many options in the DAC route especially if your computer has an optical out. As far as I know all newer Mac's have a mini-optical out in the headphone jack, my laptop does.

I think one of the more interesting choices for USB is Nixon's USBChibi (http://www.scott-nixon.com/dac.htm). I'm tempted to add that one to my home system. My previous DAC purchase ended up driving headphones at work. I started a thread to list as many sub $400 DACs as I could find at the time. If anyone can add to it...

DAC's for under $400 ??