RAM manufacturers can change specifications faster than we can adjust our recommendations, but even the most basic RAM can be overclocked by some amount.
There is only one defined "standard" voltage for DDR2 modules: 1.80 V. Nonstandard modules rated at higher voltages are supposed to be detectable at the slower settings needed to make them work at 1.80 V. That is to say, a higher specified voltage denotes a "manufacturer approved" overclock setting.
The good news is that 2.1 V is safe for nearly every module on the market, so modules that lack a manufacturer-approved overclock rating can still be used at higher voltage to stabilize somewhat higher clock speeds or lower latencies. The only way to know how far any module can go is to test it, and a few members of our Forumz follow a constantly-changing list of low-cost parts.
Recommended BIOS Settings
Anyone not fully aware of basic BIOS settings should read and fully understand our BIOS for Beginners article before making any attempt at overclocking. From there, performance favors finding the highest stable CPU speed before moving on to RAM optimization. In order to keep this guide reasonably brief, we'll look at the hardware recommended in this article's previous pages.
General steps to improve overclocking stability for all recommended boards:
Disable "Spread Spectrum" settings
Disable any smart fan speed settings
Set PCI Express to 100 MHz
P965 Chipset Settings For Intel Processors
Intel Enhanced SpeedStep Technology (EIST) and C1E Enhanced Halt State may fight for control of the processor's speed and voltage, and should be disabled. Other unused features, such as Virtualization Technology, should also be disabled; in general, if you don't know what it is, you're probably not using it.
Overclocking the CPU also overclocks the chipset's Northbridge (MCH), and increasing its voltage to 1.55 V will aid in motherboard stability. The remaining chipset and bus voltages could be left alone, but choosing the "middle" setting may offer some additional benefits with minimal risk.
Dual-channel mode allows a pair of DDR2 modules at 533 MHz data rate to provide the same bandwidth as Intel's FSB1066, and keeping memory at a 1:1 ratio with the CPU will assure adequate performance up to the overclocking limit of the CPU or RAM.
For FSB533 Celeron D chips, this means DDR2-533 can actually be underclocked as low as 266 MHz data rate without a significant performance loss. Doubling the CPU to FSB1066 - though impossible - would only bring the RAM back up to its rated speed.
For FSB1066 Core 2 Duo CPUs, this means that DDR2-800 could be underclocked to 533 MHz data rate without a significant performance loss, while increasing the CPU to FSB1600 - though difficult - would only bring the RAM back up to its rated speed.
Using the CPU and DIMM voltages recommended in earlier parts of this guide, the Celeron D 331 should easily handle a jump to 150 MHz bus (FSB600), and the Core 2 Duo E6300 should likewise accept a boost to 333 MHz bus (FSB1333). Stability testing should be followed by a small increase in bus speed (around 8 MHz) and further testing, until a speed is reached where the system crashes, locks, or otherwise becomes unstable. When instability is found, back down the bus frequencies in small steps (around 4 MHz) until the system is found to be 100% stable again.