AddOn JEDEC Standard 512MB DDR-266MHz Unbuffered Dual Rank 2.5V 184-pin CL2.5 UDIMM

Double Data Rate (DDR) SDRAM memory is a product built upon the foundation of older PC100/PC133 memory technology. DDR SDRAM memory can do two operations per clock cycle, unlike SDRAM memory which only supports one operation per the computer's clock cycle. By having DDR technology, you are doubling the memory bandwidth over the corresponding single-data-rate SDRAM.