In many cases, DDR2 1200 and DDR2 1066 will both work in the same motherboard.
There are some similarities and differences between 1200 and 1066 memory sticks, and many motherboards will accept either one. Finding out for sure is just a matter of consulting your motherboard's manual.
Similarities
Both memory sticks come in the common DDR2 (Double Data Rate 2) and 240-pin format.
Differences
DDR2 1200 is also known as PC2-9600, while DDR2 1066 is known as PC2-8500. They run at slightly different speeds; 1200 Mhz and 1066 Mhz, respectively.
Compatibility
Chances are very good that a motherboard compatible with DDR2 1200 will also accept DDR2 1066, because they are only one step apart, and many motherboards can utilize multiple types of memory sticks within a short range. However, this is not definite, and you need to consult the RAM specification section of your motherboard's manual to be sure.
Online Manual
If your manual is not handy, you can always retrieve it by entering your motherboard's make and model into a search engine and finding it online.
Caution
They both need to be DDR2 and not DDR3 or regular DDR, and they need to have the same number of pins to fit in your motherboard (this is typically 240-pin for these memory sizes).
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