We already found out in part one of our power saving guide that an upgrade from two to four memory modules will increase the power requirements by approximately 8 W for DDR2-800 systems. Now we'll look into the differences of various speed grades and popular timings.

We used two sets of DIMMs for the tests. The first was a pair of Corsair XMS2 6400C3 DIMMs, which support DDR2 speeds between 533 and 800 at very quick timings of 3-3-3-8 (DDR2-533 and DDR2-667) and 3-4-3-9 (DDR2-800). For DDR2-1066 testing, we used Corsair's Dominator XMS2 memory, rated for PC2-8888 speed (DDR2-1111), and overclocked it to DDR2-1066 speed at 5-5-5-15 JEDEC default timings.

The operating voltage and clock speed have the most noticeable influence on the power requirements of semiconductors. DDR2 memory is specified to run at a nominal voltage of 1.8 V; we used this settings for the specified DDR2 standard between 533 and 800, but we had to apply a higher voltage of 2.1 V to ensure system stability at DDR2-1066 speed. We also had to increase the operating voltage at DDR2-800 when running at ideal timings of 3-4-3-9, up to 2.0 V.

It's good to know that DRAM memory generally is downwards-compatible: if your system is designed for up to DDR2-800 speeds, it will typically work with DDR2-667 or DDR2-533 DIMMs as well, just at a lower speed. However, you should make sure that the memory parameters are set to auto-detect, so the BIOS can recognize the slower modules and throttle back clock speeds. You can also purchase DDR2-800 memory for an older solution that doesn't run faster than DDR2-533 or 667 speeds. This makes sense if you plan to upgrade your motherboard/processor in the near future.

We have produced several articles in the past that compare different memory speeds:

• Corsair's PC10000 Super-Ober-Uber-RAM Sets Sails (2007)
• Corsair's XMS2 Dominator: The World's Fastest DDR2? (2006)
• Tight Timings vs High Clock Frequencies (2006)
• How Much Memory Do You Really Need? (2005)
• Pushing Your DDR2 RAM To The Max (2005)