This memory Ram works for Dell Optiplex 990 computer?

No, this memory RAM upgrade is specifically compatible with Dell Poweredge Sc1425 (1425Sc) servers only.

Questions & Answers: CMS 16GB (4X4GB) DDR2 5300 667MHZ ECC REGISTERED DIMM Memory Ram Upgrade Compatible with Dell® Poweredge Sc1425 (1425Sc) For Server Only - B50

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CMS 16GB (4X4GB) DDR2 5300 667MHZ ECC REGISTERED DIMM Memory Ram Upgrade Compatible with Dell® Poweredge Sc1425 (1425Sc) For Server Only - B50

Computer Memory Solutions guarantees compatibility with models provided in description.
Memory Type: ECC Registered DIMM, 240-Pin
Speed: DDR2-5300, 667 MHz
4 Piece Kit
Product appearance may vary. Picture for reference only. limited lifetime warranty when bought directly from computer memory solutions