Business Desktop dc5100 (Microtower) 1GB PC2-4200 DDR2 RAM Memory for Compaq Business Desktop dc5100 (Microtower)-Elixir 1Gb 1024MB-533MHz DDR2-533 PC2-4200 240pin DIMM 1.8v. These are usualy used in dual channel Please Order Qty 2 for Dual Channel use.