Memory Bandwidth can be a daunting subject in the world of PC hardware especially for beginners. This article is not intended to make you an expert, it will, hopefully, help you better understand memory bandwidth and the role it plays in your system's performance. It could also save you some money when selecting memory for your system.

Memory Speed:

Technically, the DDR memory bus runs at memory-bus clock rate of 100MHz for PC1600, 133MHz for PC2100, 166MHz for PC2700, 200MHz for PC3200, 233MHz for PC3700, and 266MHz for PC4200. However, each DDR memory module and memory chip run at an effective (data) rate of 200MHz, 266MHz, 333MHz, 400MHz, 466MHz and 533MHz respectively. The computer industry has adopted a practical convention of just referring to the data rate as the DDR DIMM speed. So, PC1600 DIMMs are said to run at 200MHz, PC2100 DIMMs at 266MHz, PC2700 DIMMs at 333MHz, PC3200 DIMMs at 400MHz, PC3700 DIMMs at 466MHz and PC4200 DIMMs at 533MHZ.

Explanation:

With all things equal, a stick of DDR memory capable of running 2-2-2-5 memory timings will make the computer operating experience seem faster than a DIMM which can only run at 3-4-4-8. This is because of the fact that when the memory receives an instruction, retrieves the data, and sends it back out in less time.

Most DIMMs that run tight timings, such as PC3200 and PC3500 modules, have to run the memory at lower MHz than the front side bus to maintain the tight timings. However, when overclocking the processor to extreme speeds these DIMMs are bandwidth limiting the processor unless you loosen the timings so that the memory can run in sync with the front side bus or CPU external frequency.  The signifigance of this is that when the processor requires a great deal of bandwidth, the CPU will have to wait for another clock cycle before being filled, as the memory is just not fast enough to keep up at the same pace. Having a large pool of bandwidth is great when you're working with applications that process a lot of raw data, such as Photoshop and databases.

The other point of view is that CAS2-rated PC3200 and 3500 memory can make up for the lack of bandwidth because the memory has a lower latency that in effect moves data between the CPU and memory much faster. Programs that do not require a large amount of bandwidth tend to benefit more from quicker data transfers between the memory and the rest of the computer; such as games and 3D applications.

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