Case
The Case is probably the only part which is based almost solely on personal choice. It houses all of your components. The Case comes in many different standards but the two most common are ATX and BTX. ATX is a standard form factor which you basically see everywhere. All tower cases are going to be an ATX form factor. There are of course several variations of these form factors. There are Full ATX cases which are actually quite large and not seen too often. Midi ATX cases are the most common and 
come in a manageable size. These two variations can house a full ATX motherboard (the normal size) and in the case of the full size ATX case, even extended ATX motherboards (these are uncommon for now). However, micro-ATX cases can only hold the micro-ATX motherboards which are a bit smaller than full ATX motherboard in terms of height. A micro-ATX form factor can fit in any other ATX case. BTX cases are smaller, horizontal cases which are usually meant for media centres. They can only work with BTX motherboards and you need a special processor heatsink which intel an supply. The BTX cases are specially designed to fit into smaller spaces though the technology is a bit limited at the moment. It can be found in Dell, Lenovo, HP and Compaq computers. The main focus in this article will be ATX cases.
come in a manageable size. These two variations can house a full ATX motherboard (the normal size) and in the case of the full size ATX case, even extended ATX motherboards (these are uncommon for now). However, micro-ATX cases can only hold the micro-ATX motherboards which are a bit smaller than full ATX motherboard in terms of height. A micro-ATX form factor can fit in any other ATX case. BTX cases are smaller, horizontal cases which are usually meant for media centres. They can only work with BTX motherboards and you need a special processor heatsink which intel an supply. The BTX cases are specially designed to fit into smaller spaces though the technology is a bit limited at the moment. It can be found in Dell, Lenovo, HP and Compaq computers. The main focus in this article will be ATX cases.There are still a few things you need to look at before buying a case.
First is checking what type of cooling specification it can handle. The recommended standard is the Chassis Air Guide 1.1 standard which basically requires there to be duct over where the CPU would go to allow clean air to reach the CPU. It also needs a vent over the Expansion slots. Other things you need is to make sure the Case can take at least a 120mm fan at the back of the case and a recommended 120mm fan at the front. Other fans might be needed if you have a high specification computer. Some cases may include the fans and others may need you to purchase them. I personally recommend having two 120mm fans preferably with ball bearings to give them long life.
Other things to look out for is the number of expansion slots provided at the back of the case (listed on the diagram below). The usual number is 7 but other come with more. You need at least seven or four if it is a micro-ATX case. You could also check if the number of internal and external drive bays will suffice for your requirements. The external 5.25" slots house DVD drives and fan controllers while the external 3.5" slots house card readers and floppy disk drives. The internal 3.5" drive bays are for hard drives. Check that all of these are enough not only currently but also for future expansion. Finally, You should buy a case which has front USB and Audio ports for easy access, but again this is personal preference. Below is a labeled diagram to show you all the different parts of the case (please not that this is a micro-ATX case, other ATX cases will have more front bays, usually 4):
