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Memory

Just like any other components in your PC, memory modules come in a variety of speed, sizes, types and forms. Memory size / capacity range from 4 MBs to a whopping 1 GBs per module, but for the most part you will using 256 MB or 512 MB modules. Of course, having more memory is always good, but too much is not always a good thing. Most users won't see any performance improvement above 512 MB, since only the most demanding games and applications uses more memory than 512 MB. Knowing this, you might be tempted to purchase a single 512 MB module, but it's actually better to use two 256 MB module. Why? In most cases, a 512 MB module works slower than a 256 MB module. So if you're really want performance, you'll get a little more by paying a little more - two 256 MB modules cost a little over one 512 MB module.

Types of Memory

In general, there are three types of memory available today: the old standard SDRAM, the current popular choice DDR-SDRAM and the newer DDR2-SDRAM. If your PC is still using a Pentium II / III or a very old Athlon / Duron, you're probably using SDRAM. Since this type of memory is not in production anymore (or produced in very small numbers), they are very limited. In fact, while they are slower than DDR-SDRAM, SDRAM are more expensive. So, if you're planning to upgrade your SDRAM memory, don't. It's actually will be much cheaper (memory wise) to upgrade to DDR-SDRAM. You will need a motherboard that supports DDR-SDRAM since these memory standards all use different slots - you must install a SDRAM module in a SDRAM DIMM slot and this is also true for DDR-SDRAM and DDR2-SDRAM. Some motherboards come equipped with both slots. If you have one of these, only use one type of memory - you can't mix both.

Various memory types and speed

SDRAM
 DDR-SDRAM
 DDR2-SDRAM
100 MHz
 PC100
 PC1600/DDR200>
133 MHz
 PC133
 PC2100/DDR266>
166 MHz
PC2700/DDR333
200 MHz
PC3200/DDR400
 DDR2-400
233 MHz

DDR2-533
266 MHz

DDR2-667

DDR2-SDRAM arrives on the scene alongside Pentium 4's new 775 socket. Most new motherboard use this new memory standard requiring a new slot type. Unfortunately, the first generation of DDR2-SDRAM doesn't offer any performance improvement over the current PC3200 DDR-SDRAM. Only the fastest DDR2-SDRAM rated at 533 and 667 MHz offer real improvements. Since DDR2-SDRAM is only used on the new socket 775 Pentium 4s, it would be better to choose these faster DDR2 memory modules, especially if you're going to use the 1066 FSB Pentium 4. Remember, these modules don't come cheap, so if you're concerned about budget, it might be better to choose the equivalent Athlon 64 processor, since they still uses DDR-SDRAM memory.

It's All in the Speed and Timing

This paragraph may interest those who want to know more about memory performance. Just like your processor, memory has a rated working speed. That's why we have ratings such as PC100, PC133 for SDRAM, PC1600, PC2100, PC2700 and PC3200 for DDR-SDRAM, and DDR2-400, DDR2-533 and DDR2-667 for DDR2-SDRAM. These numbers indicate what their rated speeds are, in SDRAM and DDR2-SDRAM it's in MHz. For DDR-SDRAM, these numbers actually signifies the estimated memory bandwidth in MBs, so in terms of speed, a PC1600 memory module works at 100 MHz, while PC2100 works at 133 MHz, PC 2700 at 166 MHz and PC3200 at 200 MHz.

Once again, speed in not everything. There is also what's called timing to consider. Every memory works very much the same, they require several steps when accessing data to actually retrieving it when the processor requests it. First, they must look for the data and then retrieves them. The time to retrieve (and write) data is very quick and generally doesn't vary that much from memory to memory. It's the looking part that interest most people and rightly so since this affects much of the memory's performance. Timing in general is measured in latency - how much cycle the memory will spend looking for data. In short, the less latency, the faster the memory. As memory speed goes up, latency also increases. But wait! This means it is actually running slower! You're right, but this penalty is usually hidden or offset by the increase in speed. In fact, this is the main reason why a PC3200 DDR-SDRAM in most cases can outperform a DDR2-400 - the DDR2 module has a higher latency but works at the same speed. You're only going to see any performance increase over PC3200 on DDR2-533 and DDR2-667 modules.

Two Modules is better than one

Newer processors and motherboards use a dual channel memory controller. These processors and motherboards retrieve and write data to two memory modules at the same time, thus in theory doubling the performance. AMD uses dual channel memory controller in all of their socket 939 processors. For socket A / 462 AMD processors, you need to use NVIDIA NForce 2. Intel have used dual channel memory controller since their Granite Bay chipset for socket 478 and you could find the same controller on motherboards using Intel 865/875 chipsets and of course, the latest Intel 915/925 chipsets for socket 775 processors. Other third party chipset manufacturers such as SiS and VIA also provide dual channel memory controller equipped chipset for Intel's Pentium 4. To fully utilize the capabilities of dual channel memory controller, you will need two identical memory modules installed in different channels. Of course, this means they must have the same rated speed and timing. This is another reason why I recommend using two 256 MB modules instead of one 512 MB module. We could even have more performance this way.

So in retrospect, this is what you should consider when choosing your PC's memory:
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