A MotherBoard - A motherboard is the brain of the computer and is what you build everything onto - the base of your mining rig. The main feature you are looking for in a motherboard is the number of GPU slots it has as this will determine how many graphics cards or GPU's it can fit - and in the end your total hashing power.
Graphics Card - Pick your GPUs! Some graphics cards cost a fortune but don't hash much whilst some are more reasonable but use more power. Ultimately its a balance between how powerful you want your rig to be and how much you want to spend - but a word to the wise - make sure you pick an efficient GPU.
Hard Drive - You need a hard drive to store your operating system and your software for mining.
RAM - or Random Access Memory - this is a basic component in any computer is basically a scratchpad for jotting down calculations and recalling information quickly in a computer. 4GB should do the job
- A PSU or Power Supply Unit - Power supply units come in many sizes and this can trip some people up when they are looking at calculating what size they need. You need to sum up the power consumption of your GPU and all the other components and make sure your power supply has the capability to supply more! So if you have two GPU's that consumes 220 Watts and other components that need 250 Watts then you can get away with a 750 Watt power supply unit as the total power needed is only 690Watts.
- A Case - Again this can be quite a difficult choice to make as it will depend on your GPU's and whether you are using GPU risers. You don't want components sitting on top of each other as there is a fire hazard potentially.