If I may also add:

You can use Sawtooth/Gigabit Ethernet CPUs in a Digital Audio without any changes at all. Apple System Profiler may say it is running slow but other programs can generally detect it properly. (I have personally tested this, including changing the resistors and running benches) The only issue is that you are running a really slow CPU and the bus gets dropped down to 100MHz but it is good for testing a suspect motherboard or CPU.

I have not tried in a QS, but the QS is similar enough to teh DA that it *should* probably work. ymmv