Supported RAM Frequency of Motherboard and CPU

If some parts of the processor/memory sub-system can run along at a clock of 1600 but other are limited to 1066, then it will all run at 1066 (the speed of the slowest) so there is usually little to gain from having some components that can run fast (likewise, they are unlikely to make things slower either).

If everything can negotiate the higher speed then tasks where the main bottleneck is main memory bandwidth will run quicker as more data can be shuffled over the bus in a given amount of time. In reality most tasks don;t saturate the processor<->memory bus most of the time as tight inner loops usually operate of datasets that fit in the processor's cache so the need to access main memory isn't there for chunks of the time so doubling the clock will not double your systems performance (it will improve it slightly, but other bottlenecks will minimise the benefit).

There is one issue that might mean you are better getting the slower memory - running at different speeds might slightly alter the latency timings supported and voltage range requirements so if you get faster RAM make sure it is rated as compatible with the slower speed, just in case.

In the days of yore matching clock speeds could be more important. Some old 486DX3 chips would run at 33x2 if they found a 33MHz bus or 25*3 if they found a 25MHz bus - depending on what you were running and how much cache the particular chip had one or the other would be better. Sometimes (a Mandelbrot calculation loop for example) the 25*3 would be faster as the CPU could operate on register values and cached data at 75MHz rather than 66MHz, but for some tasks (say, a video encode operation) the 33*2 would be faster as it could perform bulk access to/from main memory (or off-chip cache) with a 33MHz signalling rate instead of 25MHz). There are similar effects at play with modern CPUs but they are not nearly as pronounced (so unless you are a hard-code speed freak for who every 0.1% counts don't worry about it) - modern CPUs have much finer grained control of their external<->internal multipliers so the difference won't be nearly as much as the 33/25 difference, and with their on-board memory controllers, more intelligent pipelines with duplicated core blocks & out-of-order-execution potential, and multiple cores, they can be far brighter about doing other things while waiting for one particular operation's data to arrive from off-chip.

I'd like to comment on the RAM frequency (1333/1600/etc) part. Generally, the best stick is the one that has the ideal combination of:

• lowest timings
• highest frequency
• lowest voltage
• lowest price
• being compatible with your motherboard.

But the first 3 factors are not set in stone. For example, if for the same price, you can get:

1. a stick of 1333mhz ram rated at 9-9-9-9 at 1.5V
2. a stick of 1600mhz ram rated at 9-9-9-9 at 1.5V

Stick #2 is the better stick here. Because if you "slow it down" to 1333mhz, you may be able to run it at better timings such as 8-8-8-8, or at 9-9-9-9 with a lower voltage, 1.4V perhaps, or just run it at 1333mhz and call it a day. They're practically the same chips, just tested to perform at the stated minimum specs. In other words, don't give up a good sale because it's a 1600mhz stick!

Compatibility is not set in stone either! If a 1600/9-9-9-9 stick doesn't run at this speed on a motherboard, it may actually run fine at 1333/9-9-9-9. Just like the 1333 stick of the same brand would. Of course avoid any stick you know beforehand may not be compatible.

And that is why most RAM default to 1333mhz in the BIOS: for best compatibility. It's often up to the user to configure it optimally (higher frequency, lower timings, or lower voltage) as per the rated specs, if he so desires.

Example

You can use CPU-Z to figure out the rated specs at different frequencies. Below are the specs for my ram module, officially rated 1600mhz, CL-9-9-9-9-24, 1.6V. This JEDEC table is embedded of the RAM chip itself.

As you can see, the official specs match the column for 1600mhz (actually 800mhz, remember DDR stands for double data rate). If I were to run the ram at 1333mhz (666), I could safely set the BIOS to run the RAM at 1.5V instead - in fact I should since anymore is wasted heat. At around 1200mhz, I could safely lower the timings to 8-8-8-8-22.

Now you may ask what timings could this particular ram achieve at 1333mhz and 1.6V? Unfortunately, that falls in the realm of the unknown (or the overclocking). In this case, it would be much safer to buy a chip that guarantees 1333mhz, 8-8-8-8-24 at 1.5V or 1.6V.