Memory is the best upgrade for an old computer. Most of the time, the best method for increasing performance is adding more Random Access Memory (RAM) modules. This is because newer Operating Systems and programs usually require more memory to operate effectively, and older systems with less memory usually causes a bottleneck when people install new games or a new OS.
Key Memory Factors:
The type of memory is important because the type and speed much be supported by both your CUP and your mobo. The most common types of modern RAM is Double Date Rate (DDR) 2 and 3 (DDR2 and DDR3). If your motherboard supports DDR3 memory, DDR1 (or simply just DDR) will not fit, and DDR2 memory wont work because the voltages are different and it wont be supported (this is dangerous as it can damage your system).
Obviously the rate of transfer of information. DDR2 800 megahertz and DDR3 1600 megahertz. This is vital information when one is planning on overclocking their CPU. At the time of this article, DDR2 800 delivers just as much performance as DDR3 1600, even though is much faster. This however will be discussed later when I go over overclocking.
There is quite a bit of information pertaining to memory speeds, but it is mostly pertinent to overclocking and will be discussed in great detail later. For your first build, ensuring the memory type is supported by the CPU and the speed matches the motherboard's memory standard is more important.
Essentially the time it takes for the RAM Module to respond to a data request. Lower timings equal better performance. Explaining each different type of timing is irrelevant, just know that the lower the better. If you want to know more, Google is your friend.
However, in most benchmarks, the differences between higher timings and lower timings is usually negligible and only offer a slight performance increase. It isn't recommend, however, to save money by buying the highest timings, but not necessarily worth it to spend the extra money on the lowest latency. The middle ground will usually give you the best bang for the buck.
The amount of memory in a kit or stick. Most kits run dual or triple channel depending on what the CPU supports, meaning you will have two or three sticks of memory. Most builds usually have 4gb to 6gb using sticks with 2gbs of memory each. The more the better, but make sure your CPU supports dual or triple channel, or 2 or 3 sticks. If your CPU supports dual channel but you have 3 sticks of memory, then the performance will be negligible, and possibly even hurt your performance despite the memory increase. Important to know especially if you plan on upgrading later on down the road.