The first thing that we are going to discuss is understanding your machines compatibilities. Before you customize, upgrade or replace any parts of your machine, you must first research your machines specs. All machines have varying degrees of differences in there motherboards, and even thier RAM. You must pick a mother board carefully, according to wether or not you will want or need to expand on it later. You must also bear in mind that some motherboards have no built in video, audio, or internet connections, and will therefore purchase and equip them yourself, according to your own preferences.
As you can see, motherboards are quite different from one another as is RAM shown below.