DIMMs

The DIMM is a newer designed memory module with a 64-bit memory pathway. SDRAM comes in DIMM format with 168 pins, and DDR-DRAM comes in DIMM format with 184 pins. 

An architectural difference is the pin designs. With 72 pin SIMMs, the electrical contacts (pins) are on both the front and back, but are hooked together, so you only have a total of 72 actual contacts. With DIMMs, the front and back are kept separate, meaning you have 144 total contacts. These contacts are more like little pads of metal than pins. The first DIMMs only had these 144-pins and were sometimes called “Small Outline” (SO) DIMMs. The 168-pin layout is the most common. There are additional pins added to the DIMM, which keeps the DIMM from being confused with the SIMM.

They come with choices. They come in either 3.3 volt or 5 volt designs. The also come unbuffered or buffered. This leads to 4 choices. The most common is the 3.3 volt unbuffered, although you need to consult your motherboard’s documentation to determine the type you really should have.

RIMMs

Quickly, the RIMM is the packaging style used for Rambus RDRAM. It is very similar to a DIMM, but uses different pin settings. The word RIMM is actually a trademark of Rambus, and used to refer to a module using Rambus technology.

Connectors

Memory modules come with both gold connectors and tin connectors. Older motherboards usually use the gold connector, while the newer systems use tin connectors. You can tell by simply looking at the SIMM or DIMM and checking out the color of the connectors. Also look at the SIMM sockets. Make sure you use the type of connector your board uses. If you mix-match them, over time a chemical reaction will take place between the connectors. The product of that reaction can cause unreliability and errors.