Cases

Choosing a case for your PC can either be a big deal, or something you overlook. After all, your case is what most people will see when you show them your PC.
Like previously mentioned in the CPU/Motherboard section, cases come in different sizes to accomodate for different motherboard sizes. Full or Mid tower cases can accomodate all sizes of motherboards, from E-ATX to ITX. However, if you need to save space and go for a lower spec build, you can get M-ATX cases or even smaller, and get an ITX case.
You may be thinking a case isn't a big deal, however just like with any other components, cases can offer performance increases. Some cases may offer extended watercooling support, others may have a very high airflow design, keeping all of your components cooled efficiently.
The case you choose is entirely up to you, whether it be based off the number of fans that can be connected to it, or size, choosing a case is entirely up to the user and their needs.