|The questions you're asking show that you don't know how to properly configure an Intel system, let alone overclock one. These topics have been written about ad naseum in these forums (& others), but I'll give you a brief explanation.|
On all Intel systems (other than the P4), the optimal setup for best performance is to run the CPU & RAM at the same frequency...in other words, 1:1 ratio. To accomplish that, you need a basic understanding of FSB & RAM ratings.
Intel's FSB is "quad-pumped"...DDR RAM is "double-pumped". These quad/double pumped numbers are theoretical...they are NOT the true speed (aka frequency). To determine the frequency, you need to divide by either 4 (for the CPU FSB) or 2 (for the RAM). For instance, an Intel 1333MHz FSB CPU runs at 333MHz frequency (1333 / 4). DDR2-667 also runs at 333MHz frequency (667 / 2). So running DDR2-667 with a 1333MHz FSB CPU is the ideal pairing for best performance.
If you run a 1333MHz FSB CPU (333MHz freq) with DDR-800 RAM (400MHz freq), the CPU & RAM are out of sync. What that means is the motherboard chipset has to compensate for the difference in frequencies & that introduces "wait-states" into the equation. These wait-states cause a decrease in performance. So to eliminate the wait-states, all you have to do is underclock the DDR2-800 RAM from 400MHz to 333MHz & you'll have the optimal 1:1 ratio again. If you want to take full advantage of the DDR2-800 RAM, overclock the system by increasing the CPU FSB from 1333MHz (333MHz freq) to 1600MHz (400MHz freq) & reclock the DDR2-800 back to to it's default speed (400MHz freq) to match the CPU.
AMD A64 systems are totally different. They don't use a FSB so none of the above applies. It also doesn't apply to P4 systems. The P4's architecture was very inefficient so to make up for the inefficiency, it's recommended to run the RAM freq faster than the CPU freq...usually 3:4 or 4:5 ratio, depending on the CPU.
As for why DDR2-800 is generally cheaper than DDR2-667, I can only guess. Since DDR2-800 is the general standard for *most* motherboards these days...supply & demand?