There are 4 classes of memory speed/timing specs that often get confused with each other. From slowest to fastest they are:
1) SPD settings. Each DIMM has a small EEPROM that the DIMM manufacturer programs with one or more sets of timing info. These "SPD" values are read by the motherboard BIOS on bootup and used for "auto" memory speed/timing setting. Each SPD set of values assumes standard memory voltage (1.8V for DDR2) and lists a memory bus speed (e.g. 333MHz for DDR2-667 speed) along with timings for that speed (e.g. 5.0,5,5,15).
The DIMM is guaranteed to work at these SPD settings, but they are usually chosen to be somewhat conservative, to minimize MB incompatibility problems. Different DIMM manufacturers seem to have slightly different policies on SPD settings. For example, Kingston is known for compatibility (perhaps at some cost in speed) while OCZ seems to emphasize performance (perhaps at some cost in compatibility).
*Important Note:* Any "SPD" settings shown by a software tool (like CPUZ) *have no relation to the actual current speed that your memory is running at* - they are just reading out the values programmed into the SPD EEPROM. You need to look at a different part of your software tool to find the actual memory speed and timings in current use (IIRC, the "Memory" tab for CPUZ or the "Chipset" detail for PC Wizard 2006).
Bottom line: SPD settings are just average or slow speed settings chosen for maximum compatibility with motherboards.
2) Premium-line DIMMs can often run at faster settings than those in their SPDs, even under standard voltage. However, you will need to enter the memory bus speed and timings by hand, using the BIOS, overriding the "auto" setting. Different motherboards have different procedures for this. One key point to remember is that any memory can be set to run on a slower memory bus or with slower timings (bigger latency values) without any problems -- it's running faster that may cause problems.
***Important Warning***: When running memory faster than the SPD settings, ALWAYS use a good memory testing program like memtest86+ and a CPU/memory stress tester like PRIME95 to verify that your system is operating without errors. Otherwise, small errors and corruptions may accumulate unnoticed and eventually ruin your data and/or require a complete reinstall of the OS and programs.
Bottom line: You should be able to run your DIMM at the advertised speed and timings w/o changing the voltage, *as long as the DIMM is rated at standard voltage (1.8V for DDR2),* but you may have to enter the speed and timing settings manually using the BIOS.
3) Manufacturer-sanctioned overclocking. DDR2 memory is currently at the "bleeding edge" of memory development. Although a number of (official and unofficial) standards have been worked out, production chips and DIMMs don't always meet those standards. That doesn't mean the chips or DIMMs are without value, especially given the shortage of higher-speed parts. For example, it's very difficult to find 1GB DIMMs that will run at DDR2-800 at the standard 1.8V. However, manufacturers can now produce plenty of DIMMs that will run at DDR2-800 at a slightly higher voltage, say 2.0V. (Remember, 6 months ago, it was hard to find DIMMs that would run at DDR2-800 under *any* voltage -- this is life on the bleeding edge.)
Thus, they may sell the DIMMs as "DDR2-800" memory, and note in a footnote, or on a spec sheet, that they are only guaranteed to run at DDR2-800 at 2.0V. However, since it may not run at DDR2-800 at 1.8V, they program a slower speed as the max speed in the SPD, so the system will at least boot successfully. This is a major reason why people note that their "DDR2-800" RAM is running at a slower speed on "auto" settings.
When shopping for RAM, for a given memory bus speed (DDR-x or DDR2-x), the closer the spec voltage to the standard voltage (1.8V for DDR2), the better quality the RAM.
*Important note:* In order to be able to run at the manufacturer's spec speed, you need to get ALL the speed specs, including the *DIMM voltage,* from the manufacturer's packaging, web site, or tech support. Then, in your motherboard BIOS, FIRST set the DIMM/memory voltage to the manufacturer's value. Second, set the memory bus speed. Finally, set the timing values.
***Important Warning***: Too high a voltage can permanently damage RAM. Such damage can be identified as overvoltage damage by the DIMM manufacturer. Different manufacturers have different voltage limits on their RAM guarantees, so BE VERY CAREFUL NOT TO EXCEED THE MAX VOLTAGE if the warranty is important to you. IIRC, Corsair's XMS2 max voltage is 2.1V. OCZ guarantees much of its high-end memory to 2.2V. YMMV.
In practice, what this means is that, for example, you shouldn't mix a Corsair module rated at 1.9V in the same system with a Ballistix module rated at 2.2V, as running at the Ballistix spec voltage of 2.2V will void your Corsair warranty.
Recently, Corsair and nVidia have introduced a standard for automatic manufacturer-sanctioned overclocking that they call "EPP". This standard uses additional info stored in previously-unused areas of the SPD memory to provide compatible motherboards with more detailed, higher-performance RAM OC profiles, including memory voltage increases.
Bottom line: Unless your RAM and MB both support EPP, you will have to enter memory voltage, speed, and timings manually in your BIOS to match "factory OC" specs of your module. As long as you do not exceed the factory-specified voltage, the manufacturer will support OCing a module under warranty. However, if you mix different modules in the same system, make sure the necessary voltage for one module does not exceed the limit (and thus void the warranty) for a different one.
4) "Real" overclocking. Finally, in secret workshops running outside any manufacturer limits, this is the real thing! Many DIMMs modules can actually run reliably at faster speeds and/or timings than in their spec. In general, the higher-end product lines are built using better DIMM PCBs and higher-spec RAM chips, and so have more OC potential. "Value" lines are not designed with OC in mind and typically have poor OC potential. However, for mature RAM technologies (like DDR now), sometimes ALL of the chips are high-spec, and thus even "value" DIMMs may be significantly OCable.
There are plenty of guides out there on how to OC, but there's one aspect people often forget: this techonology changes QUICKLY! Especially on the bleeding edge, just because a review OCed a DIMM by 50% doesn't mean you can do the same, even if you get the same part number. Manufacturers continuously make changes to the components, chips, and SPD programming that make up a DIMM, not to mention the chip-to-chip variations as fab conditions are tweaked, etc. RAM reviews even a few months old are best treated as historical documents, not as guides to today's buying decisions.
***Warning***: As pointed out in section (3) above, higher-than-standard memory voltages can and will damage the memory and/or shorten its life. Any OCing done by exceeding the manufacturer's limits on your DIMM will void its warranty, so be prepared to pay out of pocket for any failures caused by your OCing.
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