Motherboards
The motherboard is the very body or heart of a computer.
A motherboard offers you the very basic options for the customization of your components, for example you can have motherboards with dual channel memory sticks (simply put, two slots for Ram) or quad channel, or even more!
Same goes with Graphics cards, you can choose to have one, two or...why not? Even more!
Although it seems fun, you must check if your components are compatible with your MB! Every processor supports different types of SOCs, which means that an intel Xeon will probably not fit in a motherboard meant for Intel Core2Duo.
Same goes with Ram: DDR2, DDR3,DDR4 are NOT going to fit in the same motherboard!