DDR or Double Data Rate Memory:
Used in most modern computer systems. 
Rated in Megahertz for speed. 

184-Pins

PC1600 – 200MHz
PC2100 – 266MHz
PC2700 – 333MHz
PC3200 – 400MHz

Traditional DDR is used in AMD Socket 462 (Socket A), Socket 754 & Socket 939 platforms.  In Socket 939 platforms, the end-user can “pair” memory into groups to enable DUAL-CHANNEL support.  This doubles the bandwidth of memory to the motherboard BUS.
Traditional DDR is used in Intel Socket 478 & Socket 775 (LGA775 or Socket T) platforms.  In SOME Socket 478 platforms and MOST Socket 775 platforms, the end-user can also enable DUAL-CHANNEL support by putting the memory into EXACT pairs.

DDR2 or DDRII or Double Data Rate Memory REVISION 2:
Use in all new configurations from late 2004 through present. 
Intel platforms were first to introduce this new faster memory.

240-Pins

PC3200 – 400MHz
PC4200 – 533MHz
PC5400 – 667MHz
PC6400 – 800MHz

Newer DDR2 memory can be utilized by AMD Socket AM2 (940-Pins / Not to be confused with Opteron processors.)
Newer DDR2 memory can be used also in SOME Socket 478 and SOME Socket 775 (LGA775) platforms.  As with traditional DDR memory, DDR2 motherboards can also take advantage of DUAL-CHANNEL design, enabling double the bandwidth to the BUS when putting the memory into EXACT pairs.

It is NOT suggested to mix memory in Dual-Channel motherboards as this might cause instability.  Memory that is not in pairs is NOT suggest for Dual-Channel motherboards.  Not every motherboard will accept EVERY manufacturer of memory.  Even though the memory is rated for a particular speed and is 184-Pins or 240-Pins does NOT guarantee that ANY particular memory will work with ANY particular motherboard.  Be aware of HIGH DENSITY memory modules.  HIGH DENSITY has to do with the PHYSICAL chip design of the memory modules themselves.  MOST motherboards will NOT work with HIGH DENSITY memory.  There are additional variables to consider, please research the matter if there is something more specific than what is mentioned here.

This information was written by James A. Pavicic, Jr.  The information provided in this document is for informational and reference purposes only.  The information is provided “AS-IS” and is not guaranteed nor indorsed by PC Market Inc.  ©2006.