There are 3 ways to run windows apps on Apple hardware.
- BootCamp: you choose the OS when starting the computer. Native performance for both OS's.
- Virtualisation: (using VMware Fusion, Parallels Desktop, ..). Windows runs like an application in a window of MacOS - which is very convenient for productivity apps but often too slow for games.
- WINE based: (using CrossOver) Emulates Windows (i.e. no Windows installation necessary). You only install the PC Application and CrossOver fakes the necessary Windows parts around it to make it work.
Only #1 works with FSX.
#1 & #2 work with FS2004
I would use option #1 with a small Windows partition of 10 GB if FSX is the only MS app you would like to use.