The CPU determines the minimum RAM speed you can use. Intel processors report an inflated FSB value, which makes the whole thing more confusing than it needs to be.
On Intel chipsets, there is a connection between the CPU and RAM called the front side bus, or FSB. The default clock speed of this connection varies from processor to processor, but on current chips it's between 200 and 400 MHz. (On readily available processors it's really only 200 to 333.) This FSB determines several things. First, along with the CPU multiplier it determines the speed of the processor. Multiply the FSB by the CPU's multiplier to get the CPU's raw speed. For example, with an E8400 the multiplier is 9 and the default FSB is 333. That's why the E8400 runs at 3.0GHz. 333MHz x 9 = 3000MHz or 3.0GHz.
The FSB speed also determines how many transfers the CPU makes to the RAM over a given period of time. Intel processors make four transfers per clock cycle on the FSB, which is why Intel reports a FSB for their processors which is 4 times higher. As BlueAcolyte said, when Intel states a FSB of 1066, that's 1066 transfers per second. Divide that number by 4 (since the processor is making 4 transfers per clock cycle) and you have the clock speed of the FSB, 266MHz.
Finally, it determines the number of transfers the RAM can make back to the CPU. DDR RAM (including DDR, DDR2 and DDR3) makes two transfers per clock cycle. That's why it's called DDR, which stands for Double Data Rate. So the RAM speed you see on the package is twice as high as the actual FSB clock speed. DDR2-800 for example is designed to run on a FSB of up to 400MHz.
So as long as you're not overclocking, the basic rule of thumb is that you can divide the FSB Intel reports by 2 to get the minimum RAM speed. Running faster RAM won't hurt anything, but it won't provide much benefit.
If the motherboard says it supports DDR2-1066, that means it supports UP TO DDR2-1066, you can overclock higher.
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