1GB RAM Memory Upgrade for the Dell Dimension C521 (DDR2-800, PC2-6400) Guaranteed Compatible for the Dell Dimension C521 Speed up your system with up to 4.0GB of RAM! Lifetime Replacement Warranty! Will ship the same day when ordered at or before 4pm EST! All others ship next day! Fully-staffed call center to help with installation or inquiries! Call 866-335-1355 with questions or for help!