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Memory...
Also known as RAM (Random Access Memory). RAM is a form of computer data storage. Most computers these days will have about 2GB RAM. The higher the capacity of the RAM and the faster the speed of the RAM the quicker your computer will be. Having fast RAM will enable your computer to start up and shut down much quicker. Having high capacity RAM will enable you to use power applications such as games which require huge amounts of RAM. With 32-bit (x86) operating systems the maximum RAM you can use will be around 3.5GB. You can have more, such as 4GB or 8GB but your Operating System will only recognise it at about 3.5GB. If you want more than 3.5GB then you will need to use a 64-bit (x64) operating system which allows you to have higher amounts of RAM. However, you can't just keep adding more and more RAM to your PC as your motherboard will have restrictions. For example, most motherboards support 8GB RAM maximum. Also you must know what the maximum speed that your motherboard supports for RAM. Most motherboards these days support around 1066MHz.
Sources: General Knowledge, WikiPedia. Image of RAM was taken by one of our team members.