I created a Boot Camp partition of about 10 GB, and installed Windows XP on that. I also have an external drive formatted as FAT32 so Windows can read/write it, and I have several Orbiter installation folders on that. Orbiter works great on my MacBook Pro.
If you don't have (or want) an external disk, you might want to make the Boot Camp partition large enough to hold your Orbiter installation(s).
The Boot Camp Assistant is pretty easy to use, and you can create/remove Boot Camp partitions easily, which I did several times before I settled on my current configuration.
Good Luck, and let me know if you have more Mac questions.