For very basic use, 2GB of memory will allow a system to boot-up and operate at a very very basic level. For basic 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.  

How Memory Operates
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 memory are the current memory types available for all new systems. The most popular speed for DDR3 memory is DDR3-1600MHz and the most popular speeds for DDR4 is 2133MHz or 2400MHz. These speeds are handled as a native speed by the chipset and always offers the best support for multi-channel operation.

If a system is running in dual channel mode, two memory modules work together in tandem to allow for information to pass back and forth twice as fast as in single channel mode. Multi-channel mode is achieved by installing exactly matched pairs of modules into slots in the motherboard that are designed for this function. In dual channel motherboard with 4 memory slots, for example, you will see that two of the memory slots will be one color and the other two are another color. Quad channel memory is used in Socket 2011 motherboards and is an ultra high performance mode where 4 matching modules are used in a single memory bank.

Memory quality is a very tricky thing. It is very very easy to buy low grade memory. We do not sell any off-spec or off-brand 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.