About the usb dac thing, and I know that isn't what the original poster was asking - while Firewire Dacs are few, firewire to spdif converters aren't, and they are relatively inexpensive ($200), so you wouldn't be limited to a USB dac - you can get any dac you wish.
I suggest you read the set of ten interviews on Positive Feedback that I linked to in my original post. Pay particular attention to the interface most of the designer / manufacturer / engineers are using and why they prefer it. I believe it is the first question each of them ask. It's interesting reading. Most, but not all, choose USB for very similar reasons.
In practice I would completely agree with you regarding USB not actually running up against any walls when streaming audio. I was not, however, talking about robbing the processor of any speed or capacity, it is robbing the USB bus of throughput/device. A USB 1.1 bus has a maximum speed of 12mbps and USB 2.0 480mbps. The more devices that are using the same USB bus, the more limited the actual transfer rate of each device becomes. As I said, I've never run up against the wall except when using the computer to do other memory intensive processing at the same time and I don't think that had to do with the bus being taxed in those cases. My point was, if given a choice, why tax the transfer rate of the interface you are using to deliver the music to the DAC (assuming you are using USB which is very likely). If you use a firewire drive you will not tax the USB bus.
