Hardware
A big hard drive is absolutely required for photoscenery. I have much of a 4TB platter filled up but it has a very slow transfer rate. I still need more. PCAviator has awful customer service and sells DiskTrix defragmentation utility but the offline installation code didn't work and they haven't responded to emails so far.
Photo-scenery loads slower initially but supposedly game play is faster. Loading FSX with photo-scenery sometimes takes my computer 30 minutes and I'm still looking into tweaks. One recommendation is to turn off all enabled FSX scenery except where you are flying that day. The solid state drives now in 2018 are only affordable up to about 1TB before the prices go vertical. In ten years it will be different I'm sure. For the operating system and game this might be a good choice and then perhaps you could store scenery on separate hard drives? I don't know if that is the faster way and probably could give it a try myself. Bit of a computer brick wall of knowledge here.
You'll need big memory. 8GB minimum but hey it's not that expensive in the long run and ya don't ever want to run out. Go for it. I've got 16GB which is still not enough I guess.
PCAviator sells PrimoCache which supposedly allows using a fast SSD drive to act as a huge Level2 virtual memory buffer. I think that's how it works. It's another rube goldberg crutch but definitely seemed like an improvement in overall speed. The computer did 'run out of memory' and crash once when I flew Drzewiecki Design's New York City, despite what should be 80GB RAM total, but I am using a memory stick on a semi-broken exterior USB 2.0 port instead of mainlining onto the motherboard.
My CPU is only a single core Intel i3 3.4GHZ and it slows down a bit with a lot of autogen in the field of view. Remember FSX is 32-bit so it can only use one core and 4gb memory.
A good video card is required. I'm not sure which is the best value vs. speed. I did buy a nice $500 8GB GTX1070 from Microcenter but if you're going to be using VR goggles get the GTX 1080 I think. GPU prices have gone back up unfortunately and the 1070 is supposed to be pretty fast except for the very high end stuff. YMMV. HDMI cables are a faster route to the monitor for video output transfer of course. My crappy LG monitor has ugly screen-tearing and brightness-pulsing issues which I still need to figure out and change.
My Joystick is an Thrustmaster Warthog HOTAS (hands on stick and throttle) setup which was an incredibly stupid and expensive mistake. I walked into that piranha river with my wallet open and my thing swinging in the water. I still haven't been able to program most of the multitude of switches which were not made intuitive or correctly. The CH pedals took a while to break in and still have a clunk at center but they work and are essential for flying the whirlybirds. Get the good USB splitters and extensions while you are at the store. For a starter kit you can get a Thutstmaster t-flight stick for $50 with t-flight pedals for $60 so a decent combo setup is available for a hundred ten bucks or so.
I sometimes use TrackIR5 on a baseball cap but it makes my eyes tire out very quickly these days despite the radiation being adjusted to the lowest setting. It is a cool idea and device but version five still doesn't work correctly for me, at least. It's probably operator error. It usually is. The view screen jumps around spasticly at the edges of travel (but this may have to do with memory resources on shared USB ports vs. their own memory space and non-overlapping affinity masks whatever that means), the unit sits loosely on top the monitor and slips off the magnet base(tape the wire at the back to prevent it falling or moving), aligning the movement parameters takes some fine tuning skills when there already should be default standards in place, duh, there's no mesh collision properties with FSX native aircraft so your head can travel through the sides of your vehicle, and the software sometimes crashes and won't shut down until the dang thing is unplugged from the back of the computer after fishing around on the floor in the dark.
"But other than that, how did you like the play tonight Mrs. Lincoln?" [/rant]