Most gaming enthusiasts have long been do it yourself (DIY) types who buy components and assemble their own machines. Generally there are not that many components needed.
There are several hundred chassis models in the market. Our last build used the Corsair Carbide 300R which offers front panel USB 3.0 and mounts for numerous fans. The Coolermaster HAF 932 was considered before we selected the 300R.
The power supply is very important. It’s not a good idea to cut corners as every component depends on it. Modern power supplies now offer detachable cables making it easier to manage the wires with zip-ties.. We have a good background page on power supplies.
Intel changes the socket on motherboards every 2 years while AMD only changes it with the RAM cycle. There are many Z170 based motherboards available today. New AMD AM3+ boards are still being introduced. New AM4 boards are expected in 2017.
Once you have assembled all of the needed components it’s fairly easy to assemble a machine. Generally 16GB of memory is adequate for the most demanding games. DDR3 tops out at 8GB and sticks and DDR4 bumps that up to 16GB per stick. Unless you are doing some extreme electronic data processing, there is no real need for more than 32GB of memory.
The CPU should be at last 4 cores or more and relatively fast. The Intel i5 and AMD FX-8000 processors are sufficient for even the most recent enthusiast video cards.
If you are using a larger CPU cooler, install the mounting hardware on the motherboard first. It’s a good idea to also install the CPU and RAM on the motherboard before placing it in the chassis. Then the CPU cooler can be attached easily.
Hard disks are lower in cost than SSD and they offer huge capacities. Seagate recently began offering 10TB hard disks. Samsung recently began offering 4TB SSD drives. Motherboards have lots of SATA ports for storage. Recent motherboards with M.2 slots can achieve much more bandwidth than SATA.
The video card is the real beef for gaming. Enthusiast grade cards can be over $400. Such extreme cards afford the ability to play games with top rendering enabled. The move to 3820×2160 resolution however will remain away from the mainstream for serveral years until graphics cards can improve sufficiently to cope.