Dual Data Rate or DDR memory has been around for some time now and represents the trailing edge of memory technology. You have to work at buying new components that support DDR these days, but it's still in demand for upgrades or extensions to existing systems. Just for the record, we define a memory upgrade as a complete swap-out of older, slower RAM for newer, faster RAM, generally with lower latency or tighter timings, if not both. We define a memory extension as adding more memory modules to an existing installation without getting rid of the older modules.

Table 2: Common DDR Memory Speeds and Nomenclature

Note: Bus speed refers to the communications rate (in MHz) between the memory controller and the memory modules. Data rate refers to the number of memory transfers per second as a mega number (multiply by 1.048,576 or 210). Timings are in numbers of clock cycles. PC3700 and PC4800 are hard to find, though numerous forms of PC3500 and PC4400 remain available on today's market.

Entries in bold in Table 2, represent memory types that exceed JEDEC specifications. These are designed to appeal to overclockers. For DDR memory, PC3200/DDR400 is by far the most popular type still being purchased, according to all the memory vendors we interviewed for this story. All entries with labels that are not in bold are value products, with overclockable versions missing entirely or being extremely scarce.