Windows 7 installation permanently operate as the only OS? If I download and install Windows 7 onto my MacBook Pro, will it permanently operate as Windows, or will I be able to switch between it and OS X? I am downloading Windows 7 strictly to operate Microsoft Flight Simulator X.
Firstly, be sure to have a license for Windows 7, Windows operating systems are not "free to be used" as many Apple OSs.

Regarding your question, you can install Windows 7 in a new partition of your disk driver in order to have the possibility to choose each time you boot the machine with OS to use. There are a lot of guides online and also in this community that can help you to proceed with the installation of a dual boot.

You can also install Windows 7 on the top of the OS X you are currently using. This will remove the Apple system and you'll be able to use only the Windows one. As a personal opinion, I don't think this is a good option (and I also think you don't want to use this option) because the MacBook Pro hardware is design to work with Apple operative system, therefore, you may have some bugs/problems when you'll use Windows. Anyway, for sure the performance of the machine will become worse.

Another possibility, is to use a virtual machine that work under OS X and simulate the Windows machine where you can install the software you are interesting to use.
Again as before, I don't think this is a good option knowing that the software you want to install required high performance.