I'm running FSX dual screen style with an external 22" on my MBP 2.2ghz using bootcamp and windows xp, AND I'm running it off of an external hard drive over a usb cable at that.
Very high detail, 14 frames per second. Just have to keep the water effects down to 1.x high, seems to take a major hit, even when there's very little water around.
3d cockpit, plane graphics turned up all the way, full dynamic traffic, real world weather, blah blah blah, runs great compared to my 1.8ghz dual core Dell Inspiron.

You can drop the detail a little and easily bump the framerate up to 30 or so, but I like don't mind the slight sacrafice