Computer memory/RAM (Random Access memory) provides temporary data storage for the CPU while the computer is operating. You must select a Memory module which is compatible with your motherboard and processor, if you are going to overclock your processor then try to buy a Memory with a higher speed to gain better performance.
Most of the new processors support DDR3 RAM, however, some processors only support DDR2 RAM.. DDR2 and DDR3 can also be installed in dual or triple channel configuration to improve performance. Both the size and the speed (and preferably, the manufacturer) of each RAM module in a dual-channel pair should be the same.
How Much RAM do you need?
Every operating system has "minimum system requirements" for processor speed and RAM, recommended amount of RAM for the new 64-bit operating systems is 4GB, but in reality, the minimum requirements tend to be inadequate when you use other applications and particularly games. However, keep in mind that once the recommended amount of RAM is installed then adding extra RAM to your system is not likely going to improve your computers performance.
Always check the motherboard specifications for maximum RAM size and speed it can support.
Some Factors that affect RAM performance are:
RAM Latency is delay time measured in Nanosecond (ns) when the Memory trying to access or terminate access to a certain data on specific column and row. Memory Latency values indicate by four number in the format "tCAS-tRCD-tRP-tRAS". (For example; a Memory with latency of 2.5-4-4-6) The lower latency, results in better performance.
Two factors affecting RAM speed are; Bus Speed (in MHz) and Bandwidth (in bit). The mathematical formula uses to calculate the speed of a RAM. Generally the higher the Bus speed and bandwidth, the more the data transfer rate and performance will be. As long as you are not using very low speed RAM, buying the fastest RAM for your system doesn't result in noticeable performance.