Chuck,
Being realistic, you probably will need two machines. The thing is your sequencer machine won't need to be all that powerful. Any standard machine can run the DAW perfectly IF you are offloading all the work to a MIR machine.
I have been demoing MIR for almost a week now (purchased it full version today). It definitely takes one hell of a powerful machine to run it. Even if you think your machine is super powerful, MIR will change your mind really fast. I was able to successfully run hundreds of programs on this machine at full resolution/specs. The second I put MIR on it, MIR had it begging for mercy. There is simply no way my machine could handle both the DAW and MIR one the same machine unless I was doing maybe a piano solo.
You will be able to use Bootcamp with Windows 7 and get decent results as stated above in the post I pasted from Ron (who actually isn't me, i just posted his original post from the MIR thread). But don't expect a full orchestra. If you're thinking a full orchestra, I can almost with certainty tell you that you won't get it. It's not about processor speed as much as RAM specs. My machine runs 96 instruments in VE Pro at 10-15% CPU. My machine could play world of warcraft, record a live concert, and playback a full 100 instrument VE Pro session and still be at less then 50% CPU. But my memory controller is getting crushed. The memory controllers of the newest machines are the ones that are finally coming with enough to house MIR comfortably (I said comfortably lol).
If all you're looking for is 1-15 instruments, you just might be able to run it on that one box. But if you use Logic, you're screwed because you can't run Logic on Windows 7. In the end it will be cheaper to get a new machine to run Windows 7 and MIR and pipe it back to Logic. Because as Dietz and the team keeps telling me OS X version is a ways off (and let's face the reality here. Microsoft Windows 7 is actually far more stable so far then OS X anyway). MAC has always had a "crown" for being better, it's simply not true anymore I am sorry to say. They are falling to the wayside. Hope Snow Leopard can pull them out, as well as some nice updates to Logic (not FEATURES, stability updates). I'd take stability updates over features any day right now.
Lastly, knowing you have a nice machine now, maybe it will be more cost effective for you to put windows 7 on that mac pro and then get something like an IMac to run Logic since you will offload all the power onto the MIR machine. I know it will work because I still use a 3-4 year MacBook Pro with 2GB of RAM for Logic and all my plugs and then send my midi out MOL to a Windows 7 machine running MIR. Just another thought for you.
edit 1 - I just noticed you run Cubase. Therefore you're not stuck on one set platform. You should create a partition on your Mac Pro, and then install windows 7 on it, then cubase and then run your MIR demo. You'd have all your answers right there.
Maestro2be