FSX Steam is by far the best way to go as you do not ever have to worry about CD'S or DVD'S
but as others have stated you need to know what kind of system you are currently running .
FSX is both a CPU and GPU hog and some payware aircraft you have to have a graphic card with 4GB of ram and these cards are not cheap.
As you can see by my signature i have a descent system but could use a better graphic card as mine only has 2 gigs ram
The problem is in order to support a better video card you might have to update your power supply and of course your mother board has to support the correct slot that you will need.
So if you are running a system that is more than ten years old chances are you will have to update your entire system
You will want a 64 bit operating system so if your current OS is not 64 Bit I would suggest you update to windows 10 64 bit if your current mother board will support it.
I updated my system about 4 years ago at a cost of around $800 to $900
If you are thinking about updating I advise to stay away from store bought systems and build your system yourself. It is not as hard as it sounds
the bad thing about store bought system you do not have a valid copy of Windows and you are stuck with having to do a system restore if something goes wrong
If you build your system you will have to buy a copy of windows that you will own.