KOMPUTERBAY 4GB (2X2GB) DDR2 Memory for HP Workstation xw6400 xw6600 xw8400 DDR2 667MHz FBDIMM ECC Fully Buffered (an FBDIMM will not fit into a standard DDR2 socket, and a standard DDR2 module will not fit into an FBDIMM socket) Capacity: 4GB (2X 2GB) Speed: PC2-5300F 667MHz CL 5 Thermal Heatspreaders for Cooling 240-Pin FBDIMM Fully Buffered Lifetime Warranty