Let me break this down into something which does not need to go into a hardware debate.
First
Ask yourself what is X4?
I will tell you, an economic flight simulator >
what do simulators have in common?
CPU thread usage!
So, knowing this we have already ruled out every bit of hardware which is not a six core or greater (the personal experience I have a six core I would recommend 8 or greater for optimal balance)
Intel or AMD?
Once answer right now, AMD and why is that?
because whilst INTEL is faster clock for clock it is marginal relative performance difference over an 8700k vs AMD Ryzen 2700 is below 10% improvement per core something which you can close the gap with by simply overclocking the AMD.
What do you gain from AMD over INTEL?
More threads, A LOT MORE THREADS!!! for you hard earned $$$$
forget about what ever bull someone has to say about Ryzen most if not all of the early woes are rectified now and everything runs super smooth don't believe use Google and read up on Ryzen vs Intel testing post 2017.
(bonus upgrade paths as ZEN 2 will use AM4 socket just make sure you get the latest MOBO chipset available don't skimp out on the board)
Note: If you are thinking I am a AMD FAN... actually you would be wrong I have used nothing but INTEL after the early AlthonX generation such as AthlonX2, Ryzen however has put AMD back on the map for many like me and actually do go toe to toe with INTEL chips these days.
The Build
The success we have narrowed down exactly what we are going to use only one thing left to do and decide other parts which let me be quite clear with AMD MEMORY is a priority if you plan to overclock that AMD as the multiplier is tied to your memory speed. So we already know we are going to put most of our budget into the motherboard and memory .
Forget about power supply and storage in 2019 we are lucky enough to have cheap and affordable access to NVMe and PSU's so I will tell you exactly what to do.
- CPU - AMD 2700 or 2700X whichever fits your budget better keep in mind ZEN2 is coming this year with a near 40% performance improvement and it will give the INTEL i9's a bit of a roasting lucky for you ay, you can just plop that right into your existing socket because AMD are not *****.
- CPU Cooler - Don't need one if you are not going to overclock AMD has a nice beefy one in the box, however if you pick the 2700X grab a Corsair Hydra H100i Pro 240mm, if you have room in the case and your budget is still looking alright go with the Corsair Hydra H150i Pro 360mm (triple slot radiator for extra dissipation)
- GPU - This became a simple choice thanks to Nvidia - 1660tTi mid-range card or if you want a something a bit higher spec Nvidia 2070 (the 1660TI tho is a great performance vs cost with added tech from the RTX line up of cards, simply it is the most powerful mid-range card and has really good memory bandwidth using GDDR6)
Unless you want Radeon for FreeSync in which case there are no good cards right now wait for ZEN 2 and Radeon 3000 series cards or go Ebay yourself a cheap Vega64 ($500) - Mainboard - Do I even have to tell you? Any latest AM4 chipset within budget which has a beefy power delivery and heatsink over the VRMS ($270-$300 board whichever is cheaper between Gigabyte or ASUS)
- Memory, nothing SLOWER than 3000mhz, I recommend one of the G-SKILL 3200mhz DDR4 32GB kits. If comes between 3200++mhz or cost savings at 3000mhz step down to 16GB DDR4 and pick the faster speed. You can always throw another kit in later.
- Storage - NVMe 500GB/1TB Samsung EVO storage stick.
- Power Supply - ANY! Corsair power supply that is 700Watt or higher and gold rated efficiency or better.
(cheapest PSU wins just weigh up the efficiency vs available wattage pick the cheapest option which is in a 700watt spec)
Don't bother!
I already asked this question and CBJ gave me the answer, SLI/XFIRE are not supported in X4 Foundations they have not enabled multi-gpu use in Vulkan API (which is what this game is using)
I have duel AMD RX 480s in my system as of current and I tried crossfire it does nothing at all, I expect SLI to be the same.