For very basic use, 2GB of memory will allow a system to boot-up and operate at a very very basic level. For basic normal functionality of a modern operating system 4GB system memory is required.  8GB or 16GB are now the standard amount of memory in a modern system. For power users  32GB or more memory is a good option.  

Memory provides the working space for all of the programs and operating system components that are active at any time in your system.   DDR3 & DDR4 are the current memory types available for all new systems. AMD socket FM2 and AM3 and Intel Socket 1150 motherboards use DDR3 memory. All other modern boards use DDR4 memory.

Memory quality is a very tricky thing. It is very very easy to buy low grade memory. As a general rule, the more dramatic and colorful a memory module is the lower the grade. We do not sell any off-spec or modified CAS latency memory, because we have to support the sale of the package. We strongly recommend that if you are buying memory at the time you are buying a bare-bones, combo, or system from us that you buy the memory from us as well. The most common tech support call we take is on low grade memory problems.  We recommend and support all the memory we sell.