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It may sound trite, but it is true: size does matter, at least for memory. If you want to run Windows XP, 512 MB RAM is considered the minimum, but we recommend against anything below 1 GB. If you're eyeing Windows Vista, we consider 1 GB the absolute minimum and 2 GB an ideal scenario for mainstream users.
All high end and mainstream platforms utilize dual channel memory configurations, and even low-end systems increasingly do so as well. This means that two (or four) memory modules are plugged into the DIMM sockets so the memory controller can effectively double the memory bus from 64 bits to 128 bits, doubling bandwidth.
Three memory types are currently on the market: DDR, DDR2 and DDR3. The abbreviation stands for "Double Data Rate", which means that data is transferred during both the rising and falling edges of the clock signal. Although it is possible to run quadruple data transfers per cycle - referred to as quad data rate or QDR - both DDR2 and DDR3 are also based on double data rate technology.
DDR is available in speeds ranging from 100 and 200 MHz (DDR200 to DDR400), while DDR2 is specified for 133, 166 and 200 MHz (DDR2-533, DDR2-667, DDR2-800). DDR3 has just been launched at 533 MHz (DDR3-1066), with 667, 800 and probably even 1 GHz on the roadmaps.
DDR requires a standard operating voltage of 2.5 V; DDR2 runs at a nominal voltage of 1.8 V; and DDR3 requires 1.5 V. However, many products in the enthusiast space require higher voltage levels to reach their maximum clock speed or ideal timing parameters. The memory specifications are based the theoretical throughput: DDR400 is referred to as PC3200 (3.2 GB/s), DDR2-800 as PC2-6400 (6.4 GB/s) and PC3-8500 (8.5 GB/s). These values represent single channel operation.
Each memory technology has a sweet spot, which is defined by the available memory densities. In case of DDR memory it was a total dual channel capacity of 2x 512 MB. DDR2 has its ideal capacity at 2x 1 GB, and DDR3 DIMMs will quickly move to 2x 2 GB next year.
Of course, memory type and clock speed isn't everything. Memory is arranged in matrices, and accessing memory cells adds delays, referred to as latency. There are multiple parameters: a setting such as CL5-5-5-15 refers to the clock cycles for CAS (Column Address Strobe), RAS-to-CAS delay, RAS precharge and RAS (Row Address Strobe). Most users are fine if they remember that CAS Latency (CL) has been the dominant parameter.
Whether you want to tighten the timing parameters or increase clock speed, both require increased memory voltage. In case of DDR, speeds of up to DDR600 were reached with high-grade memory chips at around 3 V. DDR2 as presented in this article requires up to 2.45 V to perform with short timings and up to DDR2-1280 speeds. We expect DDR3 memory to max out at around DDR3-2000 and roughly 2.0 V, but this may take many months. A 10% overclock can usually be achieved on all sorts of memory, but don't expect mainstream or value memory to overclock really well.
As you start overclocking your memory, you will find that you are restricted to certain clock speed settings by your motherboard. The reason is that the memory clock is typically derived from the system clock speed (Intel) or the processor clock speed (AMD) - the system has to work with a certain ratio between system and memory clock (3:2 or 1:2), often referred to as the divider or multiplier. An example would be a Core 2 Duo system at FSB1066 (266 MHz): DDR2-533 at its 266 MHz speed would run with a 1:1 ratio, while DDR2-667 requires a 4:5 ratio and DDR2-800 requires 2:3. Sometimes there are ratios to allow users the selection of memory speeds above existing specifications. The traditional approach is increasing the system speed, while having an eye on the CPU clock speed, which is also derived from the system speed and will overclock. As you increase system speed you might have to adjust the memory ratio.