Random Access Memory

I would recommend having at least 8GB of RAM in a PC but if you want to be able to play every game without crashes due to limited RAM then I would go with 16GB. If you are planning to do video editing or have many chrome tabs open then I would recommend at least 16GB of RAM. Above 16GB of RAM for gaming means you won't see much if any performance increase than if you had 16GB.

This depends on what your motherboard allows and what Processor you have. Some Motherboards limit RAM speed to a certain frequency so if you choose RAM that is faster than that it means the RAM will run at the speed the motherboard allows. So check the manual of the motherboard you have or are planning to get (you can find the manual online even if you don't own the product) and find what speed RAM the motherboard allows up to.

For all modern Processors you will need DDR4 RAM. Intel processors use lower speed RAM than AMD Processors. For Intel I would recommend you to get anywhere between 2400MHz and 3000MHz RAM. For AMD processors I would recommend you to get anywhere between 3000MHZ and 3800MHz RAM. The reason for this is because AMD processors perform much better with higher speed RAM and Intel processors perform very similar with low speed and high speed RAM.