SERVER: FSB vs. RAM

Taken from: http://www.directron.com/fsbguide.html

Front Side Bus (FSB) - The Front Side Bus is the most important bus to consider when you are talking about the performance of a computer. The FSB connects the processor (CPU) in your computer to the system memory. The faster the FSB is, the faster you can get data to your processor.

System clock - The actual speed of your FSB with out any enhancements (such as double pumping, or quad pumping) on it. An often-misunderstood property of the system clock is its effect on processor speed. You see, a thing called a "CPU Multiplier" determines the speed of a processor in MHz. If you take the CPU Multiplier and multiply it by the System Clock speed you get the speed of your processor.

DDR2 - DDR2 memory has a different approach to design at the chip level than DDR. The simplest way to understand how it works would be to think that at the low level it had two chips of half the stated memory speed working in tandem together to achieve the full speed stated. So for DDR2 400 it would be something like 2 chips of DDR200 working together to achieve the full 400 speed. Notice that I say "chips" not sticks of memory. All this happens on 1 stick of memory. In other words, when calculating in tandem in respect to the FSB, just divide the given speed by 2.

Example:
Assume my quad-core, quad-pumped FSB = 1066MHz.
This means my System Clock is 1066/4 = 266MHz.
My CPU is clocked at 1.6GHz = 266MHz * CPU MULTIPLIER.
This means my CPU MULTIPLIER must be 1600/266 = 6.

Now here are my options for DDR2:
(A) 533MHz/2 = 266MHz. This is an EXACT MATCH to my System Clock.
(B) 667MHz/2 = 333MHz. This is the next highest speed available.
(C) 800MHz/2 = 400MHz. This is pure overkill since the System Clock is capped at 266MHz.

In these stated cases, (B) is actually preferred. (A) is an exact match which means sometimes its lag can adversely affect the CPU bus speed. As stated, option (C) is overkill.