i7 920 @ stock:
BLCK 133
CPU 20x133 = 2.66GHz
RAM 6x133 = 800MHz or 8x133 = 1066MHz
OC by raising the BLCK. Adjust the multipliers if you want/need (such as lower RAM multi if resultant speed will pass what your memory can handle). Keep in mind you can adjust the CPU & RAM multipliers independent of each other. These examples should give you an idea of how these settings work. And note, some motherboards don't explicitly list the memory multipliers, rather they list DDR3-800 (6x) and DDR3-1066 (8x).
3GHz
CPU @ 20x150
RAM @ 6x150 = 900MHz or 8x150 = 1200MHz
3GHz
CPU @ 15x200
RAM @ 6x200 = 1200MHz or 8x200 = 1600MHz
4GHz
CPU @ 20x200
RAM @ 6x200 = 1200MHz or 8x200 = 1600MHz
4GHz
CPU @ 18x222
RAM @ 6x222 = 1333MHz or 8x222 = 1776MHz
If you look at that last one, you will see that DDR3-1333 memory will handle up to a blck of 222 which will get you to 4.44GHz on CPU with 20x multiplier (hope you have great cooling). So there's really not much need for memory rated faster than that unless you're into some seriously high end overclocking.