How to select the right memory for your computer

Types Of Memory

The most common types of memory used in today's computers are DDR2 and DDR3, Although DDR2 has been the common standard for the last few years.DDR3 is still way overpriced for the speed gains you will recieve.


DDR2 Speeds

Standard name Memory clock Cycle time I/O Bus clock Data transfers per second Module name Peak transfer rate
DDR2-400 100 MHz 10 ns 200 MHz 400 Million PC2-3200 3200 MB/s
DDR2-533 133 MHz 7.5 ns 266 MHz 533 Million PC2-4200
4266 MB/s
DDR2-667 166 MHz 6 ns 333 MHz 667 Million PC2-5300
5333 MB/s
DDR2-800 200 MHz 5 ns 400 MHz 800 Million PC2-6400 6400 MB/s
DDR2-1066 266 MHz 3.75 ns 533 MHz 1066 Million PC2-8500
8533 MB/s


DDR3 Speeds

Standard name Memory clock Cycle time I/O Bus clock Data transfers per second Module name Peak transfer rate
DDR3-800 100 MHz 10 ns 400 MHz 800 Million PC3-6400 6400 MB/s
DDR3-1066 133 MHz 7.5 ns 533 MHz 1066 Million PC3-8500 8533 MB/s
DDR3-1333 166 MHz 6 ns 667 MHz 1333 Million PC3-10600 10667 MB/s[1]
DDR3-1600 200 MHz 5 ns 800 MHz 1600 Million PC3-12800 12800 MB/s

I have also noticed that DDR3 also has a slower latency, Latency is basically the amount of time the processor (CPU) has to wait between data requests from the memory.An example of how memory latency is usual shown is 5-1-1-1. This tells you that it takes five cycles to read the first bit and one cycle for each bit after that. Obviously, the lower these numbers are, the better the performance of the memory.

How Fast Is The Memory

The two main factors in the speed of your memory are Bandwidth (Measured in MHz) and Latency (see above), Technically comonsense tells us that higher bandwidth combined with lower latency will result in faster memory as it can transfer a lot more information (bandwidth) and the CPU will respond to it quicker (low latency).So to pick the best memory for your computer you should consider the next section.

Choosing Your Memory

You should always make sure your memory is running at least as fast as, or faster than the Front Side Bus speed of your CPU. Newer CPU's like the Core 2 Duo run at a FSB speed of 1,333MHz.They acheive this through "Quad Pumping" which means the CPU can transmit data at four times the actual FSB speed.Therfore if you take 1,333 and divide it by 4 you get 333MHz (rounded of course), As for the memory, DDR means "Double Data Rate" which inturn means it can transmit data at double the speed.So for example DDR2-667 memory has its own FSB of 333MHz (667 divided by 2).Therefore we should be running at least DDR2-667 memory with our Core 2 Duo 1,333MHz CPU.In my opinion you should run a minimal of DDR2-667 but DDR2-800 would be a better choice due to the minimal price difference between the two.I would also try to get memory with a latency of 5-5-5-12 or 5-5-5-15 (which is currently the most common) although lower is always better.This way you can spend your extra money on area's of your computer that really need it.With todays prices I also recommend you run at least 2GB of memory but if you have windows XP and are short on money you could get away with 1GB with out any troubles.

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