Input/output devices

Input devices such as mouse and key board are important too. With out them, we can't do work with our computers.
How to select a good keyboard and a mouse is very important for you not only if you play video games, but for your health. There are researches said that a bad habit of using key board and mouse can cause joints and other kinds of health problems.
Key boards are divided in to Mechanical keyboards and Membrane keyboards. Mechanical-switch keyboards use real switches underneath every key. Depending on the construction of the switch, such keyboards have varying response and travel times. This kind of key board is more comfortable and easy to use, and it is built with body mechanics.
Famous brands are Razer and Logitech. Razer is more for gaming, Logitech is more for commercial users.
As a huge game fan myself, mouse and keyboard are important for me. If a mouse is not comfortable to use or the sensitivity is too high or too low, it will not be pleasant to use.

Output devices such as a headset is needed too. For desk tops there are problems that there is no built in speakers. So for audio purpose, a head set is needed. Headset price can vary from $10 up to $1,000. Depend on what your use is, you can pick your headset as you like. Unless you are those who insane with Hi-Fi qualities, you won't need one over $200.