Any computer that you bought around the year 2000 or later uses a type of dynamic random access memory (DRAM) known as double data rate, referred to as DDR. It refers to memory in which two data items are transmitted with each clock signal. It replaced single data rate SDRAM. (Synchronous DRAM.) DDR memory is also synchronous, and you will sometimes see DDR memory referred to as DDR SDRAM.
DDR also replaced RDRAM (Rambus DRAM), so called because Rambus Corp. developed the standard. Rambus memory offered memory bandwidth substantially greater than SDRAM. However, RDRAM was always more expensive than SDRAM, and the lower cost and equivalent throughput of DDR eventually canceled out any performance advantages offered by RDRAM.
In 2003, a new generation of DDR memory, DDR-2, appeared along with new Intel Pentium 4 processors. Because AMD Athlon 64 CPUs had integrated memory controllers, DDR2 support didn't appear in AMD processors until the launch of a new motherboard lineup in 2006 that used the AM2 socket.
DDR-2 differs from DDR in that the front side bus carrying the data runs twice as fast as the actual memory clocking speed. To whit, older DDR memory that is clocked at 200 MHz runs the I/O front bus at 200 MHz and can transfer data at 3.2 GBps. This is because the clock speed determines the speed of the I/O bus- that's why it's called synchronous DRAM.
At the same time, DDR-2 memory running at the same 200 MHz runs the I/O front bus at 400 MHz. Each I/O bus clock cycle still carries two pieces of data, but the increased bus clock effectively doubles the maximum throughput of the system to 6.4 GBps.
The various flavors of DDR-2 are confusing because the standards group for the semiconductor industry, JEDEC, has issued variants of the specifications. For example, some high-performance memory is rated to run at specs beyond the official JEDEC rating.
To hit those higher specs, computer users have to go into the BIOS to increase voltage beyond the standard 1.8 volts. When you increase the voltage you increase the throughput. We've seen people buy DDR-2 memory that manufacturers say can run at up to 1066 MHz and be surprised that it runs only at 667 Mhz speeds because memory is shipped at officially rated speeds.
At our shop, we've seen DDR-2 max out at 800 Mhz speeds at the rated 1.8 volts, yet there is an effort to create a JEDEC standard for DDR-2-1066. Some major RAM manufacturers recently announced that they are manufacturing DDR-2-1066 chips that can run at 1.8 volts.
Because not all motherboards can support higher clock speeds, the memory manufacturers need to be sure that even higher-end memory will run at standard voltages and speeds, because they don't know in what motherboard the memory modules will end up. Many motherboards are not made to be “overclocked” to higher clock rates or voltages. All this means that you need to be careful with what type of memory upgrade you buy for your machine, and if there is any confusion at all to check with an expert in the industry.