are you sure you are getting a correct reading because it should just be 1066 and would say just put 1066 in it???
when i go to about this amc at the main box without getting technical it says ddr2 667, i could not see your being different?
I am just guessing, but the Ram manufacturers probably did not want to label their DDR2 memory 666 MHz for obvious reasons.
Probably similar reasons were used for 1066 rather than 1067.