The number one thing you can do to improve the performance of your computer is to increase your memory size. I've heard this for a number of years, and I beieve it is true. Why? Let's start with speed. Currently DDR3 memory has a transfer rate of around 9,000 Mb per second. The latest SATA hard drives transfer data at about 300 Mb per second. Memory is about 30 times faster. This doesn't take into account disk latency, or the time it takes the disk to arive at the right spot on the disk to transfer the data.
When your computer runs an application, it loads your application into memory from the disk. If you open another application, another space is set aside in memory for that application, and so on until there is no memory space available. If another application is loaded after that, your computer starts to do what is called paging. The oldest application is taken out of memory, and loaded to a swap space on your disk. As you go back and forth between your applications. You swap your pages in and out of memory from your swap space. Needless to say this going back and forth to the disk slows the performance of your computer. The more memory, the less swapping. Enough said.
The only problem is when you turn the power off and shut down your computer, you lose everything in memory, while your hard disk retains all your data and applications on the disk surface. You need both memory to run your applications and hard disks to store your data.
Let's look at a typical memory name: OCZ Reaper HPC 4GB(2 x 2GB) 240-Pin DDR2 SDRAM 1066 (PC2 8500) Dual Channel. What is all this gobly-gook or should I say gobly-geek?
Let me take it apart a step at a time. OCZ is one of your top memory board manufacturers. OCZ Reaper HPC is the brand name. 4Gb(2 x 2GB) means there is a total of 4GB of memory in two sticks of 2GB each. 240-Pin is the number of pins needed on the socket of the motherboard. DDR2 SDRAM stands for double-data-rate two synchronous dynamic random access memory. What's important here is the type of technology, DDR2. DDR2 describes the DDR chips themselves, whereas PC2 8500 denotes theoretical bandwidth, and is used to describe assembled DIMMs. 1066 is the bandwidth, the higher the faster the memory. Dual channel implies that it will work with a motherboard that can transfer data on a dual bus for improve thoughput.
One other way a spec might be presented is: DDR3 2000(O.C)/1600(O.C)/1333 This is usually what you'll see as a memory spec for the motherboard. This says the motherboard takes DDR3 memory boards running at one of the following bandwidths: 2000, 1600, or 1333. The O.C. means the memory can be overclocked.
You always want two matched pairs of memory cards and thats the way their sold. No worries. DDR3 is the latest memory technology replacing DDR2 and they both use a 240-pin socket. DDR3 is faster than DDR2 for the same clock rate.
Ok, here's what you need to know. When picking out memory, you have to match the memory board to the motherboard. If you don't you'll have a heck of a mess as the motherboard may not work and you won't know why. If you look at the above motherboard spec, that motherboard requires DDR3 memory, not DDR2, and the bandwidth on the memory has to be one of the three speeds listed: 2000, 1600, or 1333. You may be able to get away with a lower frequency, but I don't recommend it. I recommend you look for DDR3 2000 memory for that motherboard.
If you look at the first example that memory is made for a motherboard that requires DDR2 memory that can handle a 1066 bandwidth. I recommend you pick out your motherboard first, and then purchase the memory called out for by the motherboard spec, not the other way around.