I've just built a gamng PC so specs and prices are still fresh on my head. For gaming, it basically comes down to the graphics card. The budget has to revolve around that. I recommend AMD for CPU as the 3600 is still the cheapest gaming performer out there. and already comes with a CPU cooler. 16 gig of ram (3000+ mhz) is minimum to be future safe. As for motherboard, Any B450 chipset will do. I suggest MSI Tomahawk Max. There is no need for PCIe 4.0 for drives in a gaming pc. Any 650watt power supply (gold rated ). A 500 gb NVME drive for the OS and maybe another 1 terabyte sata SSD for storage and your done. It will keep up with all games and will go atleast 2 generations/iterations before needing upgrades.
So, graphics card( the most expensive part), best bang for buck is the Radeon 5700x. this will play AAA games at high/ultra settings no problem. That's the highest radeon card there is and is equivalent to a Nvidia 2070(which would be a couple of hundred dollars more). If youre willing to spend more, it's all NVIDIA from here (and there's no competition so prices go steeply up). Personally, I have a 2080 Super as I game in ultrawide.
Monitor - dont bother with 4k, if they are after FPS, it has to be 1080p/1440p and at least 100hz with fast gtg(gray to gray).
As for RGB lights.. RGB Fans are probably the most prominent additons as most parts will already have rgb on it.
And your done. ?
Hope this helps. Chris.