Quick answer is the speed at what the RAM is running. It normaly 4X the CPU FSB.
FSB: 266MHz => 1067MHz
FSB: 333MHz => 1333MHZ
I will recommand you to do a google search on "memory speed explained" or else.
Note: The CPU speed is the FSB X Multiplier.
Example: FSB 266MHz X 10 = 2660MHz or 2.66GHz (my Core2Duo E6600)