Crucial 512 MB DDR Desktop RAM, 400MHz, DIMM, 2.6V

A dual inline memory module (DIMM) consists of a number of memory components (usually black) that are attached to a printed circuit board (usually green). The gold pins on the bottom of the DIMM provide a connection between the module and a socket on a larger printed circuit board. The pins on the front and back of a DIMM are not connected to each other.
184-pin DIMMs are used to provide DDR SDRAM memory for desktop computers. Standard 184-pin DIMMs are available in PC1600 DDR SDRAM, PC2100 DDR SDRAM, PC2700 DDR SDRAM, and PC3200 DDR SDRAM

To use DDR memory, your system motherboard must have 184-pin DIMM slots and a DDR-enabled chipset. A DDR SDRAM DIMM will not fit into a standard SDRAM DIMM socket. (Information about which memory technology your system uses is included in the Crucial Memory Advisor™ tool.)