DDR2 memory is a staple component of computers everywhere with a proven reliability and known speed. While it has both younger and older siblings, this particular brand sees the most use due to its stability. Memory size depends on the make of the individual set though.
Standard memory sizes for DDR2 memory include 512 MB, 1 GB, and 2 GB sticks. Larger ones are available as well, but what a given computer can take depends on its motherboard. Increasing memory helps one's computer work faster.