Computer RAM

The Computer RAM (Random Access Memory) is our short term memory. When you start an application, for example to play a video, the video player is loaded into the RAM for quick access. That is also true for the video itself, although it is put into RAM in portions. Without the RAM the computer would be much slower. The access time to the Hard Drive (our long term memory) is about 1000 times slower than RAM. On the other hand the Hard Drive can hold about 1000 times more data than our RAM.

As with the CPU, the Computer RAM has progressed in speed over the years. All RAM used in computers are SDRAM (Synchronous Dynamic Random Access Memory) and of the DDR (Double Data Rate) type. With time DDR has evolved into DDR2 and lately to DDR3, each time increasing speed and performance. See at the end of this page for comparison between DDR, DDR2 and DDR3.

As you know you cannot put an AMD processor in an Intel socket. The same goes for RAM. The DDR2 will not fit in a DDR3, because the key notch has a different location although the number of pins are the same. And vice versa of course.
Computer RAM
There are some limitation to what can be done with the RAM. First, a 32 bit Operating System can only handle 3 GB of memory (64 bit can handle much more). So it makes no sense to put in 4 GB if you are running 32 bit Vista. Second, you can only put as much memory as the Motherboard allows, normally much more than you need. Third, the RAM has to match type (DDR2, DDR3 etc) and Bus Speed.

If the Motherboard specifies DDR3-1066, then that is what you have to purchase. DDR3-1333 will work but at the same speed as DDR3-1066, limited by the FSB. DDR3-800 will probably not work.

How to choose RAM

Choosing Computer RAM is quite simple. 1 GB of RAM is fine if you running Linux or Vista Home Basic, otherwise choose at least 2 GB. More than 4 GB is usually not necessary. Since most Motherboards now support Dual-Channel, always use two memory sticks (for example 2 x 1 GB or 2 x 2 GB). What you have to double check is the RAM type. For a Gaming rig you should also have a look at High Performance RAM. The Motherboard should state clearly what memory is required, for example DDR2-800.

Brands

The brands mentioned below are all the well-known ones. Some online vendors also offer generic Computer RAM, which sometimes can be a very good deal. I usually do not purchase generic because of quality reason. If you are building a budget system (< US$750) then it is worth considering. A few dollars can go along way in you want for example a Video Card.

Corsair
Mushkin
pqi
Kingston
G.Skill
Wintec
Crucial
Patriot
OCZ

Technicals

Latency: This is a complex subject. Basically we look at Timing and CAS latency. For example the Corsair XMS3 DHX (image above) has a Timing of 9-9-9-24 and CAS latency of 9. The lower these numbers are the better.

DDR

DDR memory sticks have 184 pins for desktop computers. Laptop DDR have 200 pins. One key notch.

Name	Memory Clock	Bus Clock	Data Transfers
DDR-200	100 MHz	100 MHz	200 million/s
DDR-266	133 MHz	133 MHz	266 million/s
DDR-300	150 MHz	150 MHz	300 million/s
DDR-333	166 MHz	166 MHz	333 million/s
DDR-400	200 MHz	200 MHz	400 million/s

DDR2

DDR2 memory sticks have 240 pins for desktop computers. Laptop DDR2 have 200 pins. One key notch.

Name	Memory Clock	Bus Clock	Data Transfers
DDR2-400	100 MHz	200 MHz	400 million/s
DDR2-533	133 MHz	266 MHz	533 million/s
DDR2-667	166 MHz	333 MHz	667 million/s
DDR2-800	200 MHz	400 MHz	800 million/s
DDR2-1066	266 MHz	533 MHz	1066 million/s

DDR3

DDR3 memory sticks have 240 pins. One key notch located different from DDR2.

Name	Memory Clock	Bus Clock	Data Transfers
DDR3-800	100 MHz	400 MHz	800 million/s
DDR3-1066	133 MHz	533 MHz	1066 million/s
DDR3-1333	166 MHz	667 MHz	1333 million/s
DDR3-1600	200 MHz	800 MHz	1600 million/s

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