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66.6MHz (frontside bus)
*  10 (clock multiplier)
------
666MHz (clock speed)

Unfortunately, there is no 66.6MHz frontside bus. The closest thing there is is the 66 2/3MHz bus, known colloquially as the 66MHz bus and roughly but more accurately portrayed than the above as 66.6666666666MHz. Thus:

66.6666666666MHz (frontside bus)
*                       10 (clock multiplier)
----------------------
666.666666666MHz (clock speed)

Also known as 666 2/3MHz. Which is much, much closer to 667 than to 666.