People here present some good points to keep in mind. Here are a couple things to note about FSX to consider (and yeah, the Steam version, at least for me, has been more reliable than disc installs - I can't get the disc installs to register any longer).
1. FSX is heavily CPU bound, so a high end, modern, fast CPU will do you a lot of good there. I would start with that before upgrading my graphics card.
2. FSX is a 32-bit program, so no matter how much memory you have, it's stuck in a 4 GB limit. It uses a lot of this memory for add-ons, scenery, and sim objects like complex aircraft (which can slow down performance a lot). So you will be limited to how many add-ons or complex sceneries you can utilize. Too many can result in crashing and out of memory errors soon into a flight.
I have been a flight simulator and P3D user for many many years (more than I want to count) and MSFS is a fine and very accurate simulator. If you want realistic flight as well as beautiful scenery, and your computer can handle it, it's an excellent choice. I keep P3D around for some of the aircraft I can't get on MSFS, but that's the only reason. Now that FS Traffic from Just Flight is out, I have one less reason to keep P3D active. As with most things, which simulator to use is a personal choice based on your needs, capabilities, and preferences.
As far as scenery for FSX or P3D, I chose to go with Megascenery Earth's excellent satellite imagery. Over the years, I acquired most of the United States. It looks good from at least a little altitude (about 3,000 ft?). Since it's real world scenery, I can use it for VFR flight using landmarks. No real framerate impact, either (although having a lot of them can increase the startup load time for your flight). I attached a sample screenshot so you can see what that scenery looks like.
Hope it works out well for you, regardless of which simulator you use.