You can run Windows in OSX. Best of both worlds.

Be interesting to see how FSX is handled by a full spec macPro.
Actually there is no need. As long as you have two drives, you can dual boot. OSX on one drive for editing, XP on the other for actual computing. And yes Vista has been booted on a MacBook Pro, and a Mac Pro.
As far as gaming goes, because they have support for x86 drivers you can actually use real spec gaming cards in a Mac. Therefore I would expect a Dual Xeon (4 cores total) with 16GB of RAM, 4 Raptors in a RAID0 and a 8800GTX would crush the hell out of FSX. Now finding a Mac with a 8800GTX just isnt going to happen, a FireGL card maybe.
Cheers
Cameron