1 X 1GB 667MHz DDR2 PC2-5300 SDRAM Desktop Computer Memory for Dell OptiPlex SX280, GX520, GX280 This memory is also backward compatible with systems using PC2-4200 DDR2 Memory PC2-5300 (667MHz) Data Rate 240-Pin Dual in-line memory module Non-ECC Unbuffered 7.8US Refresh Interval (8192 CYCLES/64MS)