The Complete Guide to Buying Memory (RAM)

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The Complete Guide to Buying Memory (RAM)

Random access memory , or RAM, is a vital to a computer's performance. As the buffer zone between the hard drive and the central processing unit, RAM to a large extent determines how much, and how quickly, data can be accessed and transferred. For those who find their computer is running sluggishly, upgrading RAM is a cost-effective way to improve or even eliminate the situation.

When shopping for RAM, buyers are faced with a daunting set of specifications. At the outset, they should verify the requirements of their current computer, including its motherboard, central processing unit, and operating system. Memory upgrades must be compatible with and supported by existing hardware and software, so researching in advance both narrows the field of options and helps prevent unnecessary spending on high-performance memory that a computer is not capable of running optimally. By educating themselves about RAM specifications and those of their own computer, buyers can find the right memory to help them get the most from their computer.

Random Access Memory: An Overview

Random access memory, or RAM, plays a crucial role in a computer's performance. It acts as a buffer between the hard drive and the central processing unit, or CPU. The hard drive holds the mass of information stored on the computer, but is slow at writing and reading data. The CPU is responsible for executing logical, mathematical, and input/output tasks required to run programs. Information from the hard drive is sent to RAM in order for the CPU to process it. The greater the RAM capacity, the less often data needs to be retrieved from the hard drive, which means faster performance overall.

The Advantages of a RAM Upgrade

Upgrading RAM is a relatively inexpensive way to improve performance, particularly on older computers with smaller RAM capacities or earlier types of RAM. While the RAM included in newer computers is more powerful, advances in software seem ever more memory-intensive. Therefore, a RAM upgrade can still boost power for things like enthusiast systems, workstation applications, and more.

Whenever upgrading RAM, or building a computer from scratch, it is vital that the RAM be compatible with the computer itself, supported by the operating system, and matched to the CPU.

RAM Specifications and Compatibility

The array of technical specifications for RAM is extensive, which can be rather confusing for the consumer as they try to find the best memory within their budget. While theoretically choices abound, the set of actual options is significantly limited by what a given computer can support. Before shopping, buyers should research what their computer's motherboard can support in terms of socket type, speed, capacity, and power supply. There are Internet sites where consumers can enter their computer model and operating system and receive information about what RAM is compatible with their equipment.

RAM Type and Motherboard Compatibility

RAM comes in various types denoted by long acronyms. They differ in performance as well as pin configuration, which determines whether they fit to the computer motherboard.

RAM Type



Single Data Rate Synchronous Dynamic Random-Access Memory; accepts one command and transfers one data word per clock cycle; earlier, slower version of RAM


Double Data Rate Synchronous Dynamic Random-Access Memory; has nearly twice the bandwidth of SDR at a certain clock speed because of a double-pumping interface


Superseded DDR; best-rated DDR2 modules are twice as fast as DDR


Supersedes DDR2; able to transfer data at twice the rate of a DDR2 module and runs at lower voltage for improved energy-efficiency

Because these RAM types are neither forward- nor backward-compatible, they need to be matched to the computer's other components. Shoppers must verify what type of memory their motherboard is able to fit: SDR, DDR, DDR2, or DDR3. There are still other classification factors to be considered and matched with the computer, as discussed below.

Decoding RAM Classifications

In addition to RAM type, consumers need to pay attention to the details of memory classification, which is based on the speed at which they can run, as well as their timings. DDR, DDR2, and DDR3 follow a DDRxxx/PCyyyy classification system. The first figure, "xxx", classifies the memory chip according to its maximum supported clock speed; for example, a DDR2-800 has a maximum clock speed of 800 megahertz (MHz). Buyers should be aware that real speed is half of the maximum speed, so a DDR2-800 would actually work at 400 MHz. The second figure, "yyyy", classifies the memory module according to its maximum data transfer rate in megabytes-per-second (Mbps). DDR3-1333 memories can transfer a maximum of 10,664 Mbps, so they are labelled PC3-10600 or PC3-10666, where the "3" after "PC" indicates that it is DDR3, as opposed to DDR or DDR2.

Clock Rate and Actual Performance

The theoretical maximum data transfer rate used in classification is based on the fastest clock speeds supported by the memory. However, if the memory bus speed on a computer's motherboard is slower than the tested clock speed, the actual data transfer rate is lower. For example, a DDR3-2000 mounted to a DDR3-1600 motherboard runs at 1600MHz, rather than 2000MHz. The exception is with overclocking, manually forcing an increase in the memory bus clock, which can increase the memory module's performance on a given system.

CAS Latency

In addition to the basic classification, RAM specifications also include a series of numbers, such as "CL7-7-7-18". The first number in this series is CAS latency, the number of clock cycles it takes the RAM to access a column of data. Latency is another important piece of information needed when comparing the performance of different RAMs. Lower numbers indicate faster random-access memory, and in some cases a module with a slower clock speed but lower CAS latency is in fact faster than another with a faster clock speed but higher latency.

Form Factor

RAM form factor indicates its pin configuration for mounting to the motherboard, and must be compatible with the motherboard. Most memory modules are DIMM (dual in-line memory module), while older modules may be SIMM (single in-line memory module). A DIMM features electrical contacts on either side of the module, while SIMM has contacts on one side only. DIMMs typically have a 64-bit path, meaning that data is transferred in 64-bit units, while SIMMs have 32-bit paths.

Different form factors are specified for different types of modules, and an SO- prefix indicates that it is for a laptop computer rather than a desktop. For example, a 240-pin DIMM is used for DDR2 SDRAM and DDR3 SDRAM, while a 204-pin SO-DIMM can only be used for DDR3 SDRAM on a laptop.


While the general rule prevails that more memory is better, higher RAM capacity means higher costs, so buyers should try to realistically assess how much memory they require for their particular computer use. RAM capacity is generally measured in gigabytes (GB), and 2GB has been thought to be the bare minimum, while 4GB is more broadly recommended as a standard for frequent use of things like email, word processing, watching videos, and multitasking. These figures can be expected to rise, however, as operating systems and applications become more and more sophisticated. Between 4GB and 8GB is recommended for more intensive use, including video editing, gaming, and heavy multitasking, while above 8GB is reserved for professional users, designers, and serious gamers.

As with other specifications, buyers should be aware of their existing hardware and system capabilities. There is a maximum amount of memory that a motherboard can accept, and a maximum amount of memory that an operating system can accept.

Number of Modules

RAM is sold in single and multiple modules. Deciding how many to buy depends partly on how many the motherboard can support. In certain cases, systems require a pair or pairs of modules, and others may not require this, but function better with module pairs.


An oft-overlooked RAM specification is the power requirement, expressed in volts (V). Each type has a standard voltage, e.g., 1.5V for DDR3 and 1.7V for DDR2; however, some performance DIMMs may have higher voltage requirements. When the latter is the case, complications may arise if the motherboard does not recognise the non-standard voltage requirement and supplies the standard voltage. This is yet another reason why it is imperative that they buyer understand their motherboard requirements before purchasing RAM.

How to Buy Memory (RAM) on eBay

If you are interested in shopping for memory online, take advantage of eBay's extensive selection of new, used, and refurbished RAM devices. Start on eBay's home page and locate the search bar, where you can enter keywords like "4GB RAM &", or " DDR3 memory &". Refining your original search results helps narrow your choices; you can select your price range, preferred manufacturers, and other limiting factors.

Always read a full listing carefully, checking for details like item condition, product specifications, and postage fees. Verify that the specifications are compatible with your requirements. If you need more information, you can ask the seller a question via eBay.

Get to know the seller a bit more by looking at their feedback score and comments left by previous buyers, preferably those who bought items similar to yours. This helps you find a seller that is reliable and knowledgeable. Top-rated sellers complete a high volume of transactions whilst maintaining an excellent standard of customer service.


Upgrading a computer's memory (RAM) is one of the more inexpensive and simple ways of improving its overall performance. Random-access memory is a buffer zone between the hard drive's massive store of data and the central processing unit, whose job it is to process that information. Generally speaking, the faster and larger the RAM, the more quickly and powerfully the computer runs. This can be particularly useful for those who run sophisticated, memory-intensive programs.

There are many RAM specifications that must be considered when choosing the right module for a given computer and system. Issues of hardware compatibility are crucial, and the specifications of the motherboard must be considered in tandem with the memory's performance capabilities. Specs to watch for include RAM type, maximum speed, maximum data transfer rate, CAS latency, form factor, and voltage. Capacity is another important consideration, and largely depends on computer use and operating system.

Although buying memory may seem daunting at first, with some knowledge and research buyers can find the right memory to help their computer run optimally.

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