4GB Memory Upgrade for Dell Dimension E521 Desktop PC 4 X 1GB DDR2 NON-ECC PC2-6400 240 pin 800MHz DIMM RAM (PARTS-QUICK BRAND) Capacity: 4GB Total 4 X 1GB Speed: PC2-6400 800MHz DDR2 Form Factor: 240 pin DIMM Halogen Free; ROHS Warranty: Lifetime