Originally Posted by Chozart
I'll give it a shot. You know the CPU has an external frequency of 200MHz, and runs on a mulplier on top of that frequency (your 3700+ has a multi of 11, thus running at 2.2GHz). DDR RAM works the same in a way, and DDR 400 runs on an external frequency or also 200MHz. With everything at stock values, the ratio between RAM external frequency and CPU external frequency is 200:200 or 1:1, which we call the 1:1 divider. Now, I guess you knew all this.
When you overclock, you will increase the external frequency of the CPU. However, despite the fact that you set your RAM at, for example, DDR 400 speeds (or 200MHz external RAM frequency), your effective RAM still will go up when you increase the external frequency of the CPU! In this specific case, it will go up with the CPU on a 1:1 ratio. Thus, when you overclock your CPU 10%, the CPU external frequency will be 220MHz, the CPU clock speed will be 2.42GHz (11x220), and the external RAM speed 220MHz, thus resulting in DDR 440 speeds. (remember, we're now still on a 1:1 divider).
Now, obviously, RAM can take only so much, and often less than the CPU. Let's assume that 10% is the max for the RAM. The 1:1 divider would then limit the CPU clock to 2.42GHz. You want to get higher though! Thus, time for a divider. You set the divider my adjusting the RAM frequency in the BIOS. For example, we want to get a bit higher, we set the RAM frequency at DDR 333, or 167MHz external frequency (whichever option your board offers). Now, at stock, external RAM frequency is 167MHz, and the external CPU frequency is 200MHz, or a ratio of 167:200, thus a 5:6 divider.
Now, lets increase the CPU external frequency to 240MHz, hitting a CPU clock speed of 2.64GHz (11x240). RAM frequency will increase together with the CPU frequency, but since we now set a divider of 5:6, we will get the following outcome: External RAM Frequency = 240/200 x 167 = 200MHz, or DDR 400 speeds. Hey, RAM is back at stock speeds, with a good overclock on the CPU! I hope you understand the logic behind this now.
Now, some motherboards allow you simply to set the RAM frequency higher than the CPU frequency, but this is not really a helpful function. First, you'll need DDR 500 or so RAM to do this, adn second, there is not a noticable increase in performance when you simply increase RAM speed (it won't be able to handle more than the CPU delivers anyways).
Now, what you should try is to find the Max CPU speed (knock RAM down on a heavy divider, and crank the rig up as far you can), the Max RAM speed (lower the multi, and keep going on 1:1 until the RAM does not want anymore), and the Max HTT of yuor motherboard (lower both RAM frequency and multi and clock the crap out of it). Now, when you know those values, you can try to find your ideal overclock. Let's assume your CPU maxes out at 2.8GHz and the RAM at DDR 440 or so, while the FSB can take a beating higher than those values (thus excluding it as a limiting factor).
For your CPU to hit 2.8GHz on a multi of 11, the external frquency has to be 2800/11 = 255MHz. On a 1:1 divider, this would result in a RAM speed of DDR 510, exceeding the RAM limit. Thus, we need to set a divider. The highest external frequency the RAM will do is 220MHz, while the CPU external frequency is 255MHz. This is a 220:255 or 44:51 ratio. Ideally, we want to set RAM on this divider, but your board will not give this option. At stock, the desired RAM speed to set for this divider is 400/51*44 = 345MHz. Then we look for the next lower available setting, which is DDR 333 speed! Thus a divider of 5:6 will work in this case.