Even though I have owned Macs from the beginning, it was Amarra, Weiss, and the use of Firewire that achieved a level of performance that I have not heard in any other digital. I just think it is ridiculous that USB is being used.
I'm pretty sure Firewire's been used in Pro-audio since before adopted by PC-audio high-end manufacturers. There are implementations of USB that are better than others (USB>I2S, and asynchronous USB). As I understand it, using Firewire requires custom plug-in software that is proprietary to the device used, whereas USB can be implemented more simply and universally without plug-in software. A recent review of Empirical Audio's OffRamp and DAC in TAS (FWIW) claimed that the USB combo of the OffRamp and Empirical's new DAC surpassed the performance of the Weiss Minerva with Firewire interface. In that same article (again, you throw in as much grains of salt as you see fit) the reviewer claimed that using the Empirical USB Offramp with the Minerva improved on the direct connection of the Minerva via Firewire. Details in the Feb '10 TAS.
Rwwear - sounds like your GF needs a new MacBook :-) I actually use a Macbook, but NOT for a music server (I use my tower for that) and would not recommend the base MacBook for that purpose. Especially not older ones...they will not run the current OS (10.6) well at all. It's the same planned obsolescence with PC's - my brother and I just replaced my Mom's PC (only 3-4 years old which is a lifetime in the PC world - this is not exclusive to either platform) for similar reasons. The hardware has to keep up with the software and eventually it will clog up and bog down. The new OS for both platforms requires more RAM, more HD free space and more processor power than ever (even though streaming music is not really processor/memory intensive). FTR - I would definitely choose a MacMini or iMac as a music server over a laptop.