Again, the overwhelming majority of professional facilities use Mac computers for a reason. And no, it's not just because they can afford it. Your computer is your lifeblood in this business. It's not just a computer, it's an investment into your craft. Choose wisely. Apple is absolutely unmatched in reliability, hardware and software integration, as well as customer support, among many other things. In almost 10 years of owning and using Mac computers, I have not had a problem and will never, ever go back. Mac works both 32 and 64 - it will easily meet your needs, whatever they are. If you get an 8 or 12-core Mac pro you will be set for years to come, and enjoy virtually no issues during that time as well as stellar performance. Lastly, it is well known that ProTools works better on the OSX platform to begin with, so right there out of the gate Apple should be the first choice.
Call me a fanboy if you want, but I would not consider a facility "professional" if it had Windows machines.