PC MEMORY TYPES

RDRAM -- Rambus DRAM

 If you are using RDRAM, make sure that all memory sockets of a channel are filled with either a memory chip or a continuity module. RDRAM often has to be installed in pairs of the same type of memory chips.

RDRAM devices may be configured into single-, dual- or quad-channel RIMM modules. For dual-channel or quad-channel (4-channel) RDRAM chipsets and motherboards, memory module upgrades should be in matched pairs. For instance, to add 512 MByte of memory into a dual or 4-channel system, two matched 256 MByte modules should be inserted.
 Example of RDRAM speeds include PC 800 (PC-800 or PC800) and PC 1066 (PC-1006 or PC1066).

SDRAM -- Synchronous DRAM (JEDEC SDRAM)

SDRAM (synchronous DRAM) is a generic name for various kinds of dynamic random access memory (DRAM) that are synchronized with the clock speed that the microprocessor is optimized for. This tends to increase the number of instructions that the processor can perform in a given time. The speed of SDRAM is rated in MHz rather than in nanoseconds (ns). This makes it easier to compare the bus speed and the RAM chip speed. You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.


Example: PC-133 CL2 SDRAM = 133MHz (SDRAM, PC133 • CL=2 • Unbuffered • Non-parity • 133MHz • 3.3V )

DDR SDRAM -- Double Data Rate SDRAM

DDR SDRAM is synchronous dynamic RAM (SDRAM) that can theoretically improve memory clock speed to at least 200 MHz*. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling output. It's expected that a number of Socket 7 chipset makers will support this form of SDRAM.
Synchronous DRAM speed is measured in MHz rather than nanoseconds (ns). You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns. 
Example:
PC 66
PC100 = PC-100 DDR RAM was sometimes called PC-1600 SDRAM because of its data bandwidth (transfer capacity) of 1.6GB per second.
*comes in 184-pin DIMM and 200-pin SODIMM formats
PC133
PC2100 (DDR266)
PC2700 (DDR333)
PC3200 (DDR400)

DDR2 SDRAM -- Double Data  Rate  SDRAM 
Example:
PC2 3200 (DDR2 400)
PC2 4200 (DDR2 533)
PC2 5300 (DDR2 667)
PC2 6400 (DDR2 800)

DDR3  
Example:
PC3 8500  (DDR3 1066)
PC3 10600(DDR3 1333)
PC3 12800(DDR3 1600)