DDR memory, sometimes referred to as DDR SDRAM, stands for Double-Data-Rate Synchronous Dynamic Random Access Memory. DDR memory is based on the same architecture as the original SDRAM (single-data-rate) but it can transfer up to twice as much data in the same amount of time. DDR and the original SDRAM modules are not interchangeable since they have different pin configurations (DDR has one notch while SDRAM has two). How can I tell the difference between DDR and SDRAM? A DDR SDRAM module has 184 pins for the dual inline memory modules (DIMMs) of the desktop computer. SD RAM has 168 pins and the DDR2 SDRAM, which is the next version of DDR SDRAM, has 240. It is easy to tell the DDR and SD models apart, simply look at the number of notches on the memory card. The DDR version has one and the SD version has two.

Speed : PC2100 - DDR266; PC2700 - DDR333; PC3200 - DDR400
Form Factor: 172-pin-Micro-DIMM (Notebook); 184-pin-DIMM (Desktop); 200-pin-SODIMM (Laptop);

Generally speaking, speeds of DDR RAM can be mixed. DDR ram is backwards compatible however they will both run at the speed of the slower module. For example, you can mix PC2700 and PC2100 just fine, they will both end up running at PC2100 speeds. Depending on the motherboard, you may need to install the slowest stick in the first RAM slot.

Dual DDR means there are dual channels for memory instead of just one, it is supposed to double the memory bandwidth, so if you had 2X 256MB of PC3200 RAM you would have a theoretical bandwidth twice that the RAM is rated for. PC3200 is rated for a max theoretical bandwidth of 3.2GB/sec or 3200MB/sec. On a dual channel DDR motherboard, the theoretical max bandwidth would be 6.4GB/sec, or 6400MB/sec. The theoretical bandwidth does double, but benchmarks show a typical overall system gain of 5 - 20%.