PC Stats has posted a beginners guide about memory upgrades

Modern computer processors can perform several billion operations per second, creating and changing incredible amounts of data in a short period of time. To perform at this level, they have to be able to juggle the information they process, to have someplace to store it until it is needed again for modification or reference. Random Access Memory (RAM) can be thought of as the short-term memory, in the sense that once the power is turned off, all information stored there is not saved. All modern computers have hard drives which store data permanently as magnetic information, but even with the improved speed of today's hard drive technology. Hard drives are still too slow to keep up with the needs of the processor since it can operate on considerably more information per second than can possibly be transferred to and from the hard drive.

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