• SD Ram is actually SDR SDRAM, and DD Ram is actually DDR SDRAM, though the former does not usually have the SDR in front of it. The letters SDR and DDR stand for Single Data Rate and Double Data Rate, and the letters SDRAM stand for Synchronous Dynamic Random Access Memory.
The main difference between SDR and DDR memory is speed. There are a lot of little differences, but the main one (IMHO) that affects the user is speed: DDR can transfer data at roughly twice the speed of SDR. More speedy data rates = better performance.
Just remember, the motherboard you are using must include the appropriate chipset to support the different RAM types. They are not interchangeable.
SDR SDRAM comes in three main flavors: PC66, PC100 and PC133. Each successive number refers to the bus speed of the RAM in MHz, thus PC66 runs at 66 MHz, PC100 runs at 100 MHz, etc. SDR SDRAM has 168 pins at the connector.
DDR SDRAM has 184 pins at the connector, which is one reason you can't just use DDR instead of SDR, and comes in many different flavors; PC2100 which runs at 266 MHz, PC2700 which runs at 333 MHz, PC3200 which runs at 400 MHz, etc. In order to know which type of RAM you need, you must know what your motherboard supports.
• While the numbers on SDR SDRAM referred to clock speeds, the DDR SDRAM numbers come from the theoretical maximum bandwidth in Megabytes per second (MB/s). Thus, PC2100 has a theoretical maximum bandwidth of 2100 MB/s, but runs at 133 MHz. DDR SDRAM modules are available from PC1600 all the way up to PC4400 now, which has a bandwidth of 4400 MB/s and runs at 550 MHz
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