The Elpida 4GB DDR2 PC2-6400U-666 240-Pin DIMM is a set of four 1GB memory modules designed for computer systems. These DIMMs feature non-ECC and unbuffered memory, making them suitable for high performance systems. With a total capacity of 4GB, these DDR2 SDRAM modules have a bus speed of PC2-6400U-666 and are backed by a manufacturer warranty of 12-D0. Ideal for upgrading or replacing existing memory modules, this Elpida product is reliable and efficient for a variety of computing needs.