MIR will be available for windows vista and windows 7 in a first run (both have to be 64bit because of the amount of memory needed to be accessed), OS X might follow later.
so you could run windows via bootcamp on your MacPro and within windows MIR, but be aware the *old* intel 5000 XEONS have slow memory bus speed (at least compared to i7 processors)
if an audio host and MIR run simultaneously on the same machine will depend mainly on your soundcard drivers.
to step in regarding userland question: you probably know that GUI is not 64bit on leopard and VI/VE runs in memory space for user's applications - this makes it a little bit complicated to access more than 4 GB memory with an application needing a user interface. however a solution is under development.
christian