CMS 8GB (2X4GB) DDR3 10600 1333MHZ Non ECC DIMM Memory Ram Upgrade Compatible with Dell® Optiplex 990 Dt/Mt/Sff, Ultra - A69 at Amazon.com
Dell OptiPlex 990 Intel Core i5 2400 3.1GHz 4GB RAM 500GB HD Windows 10 Home 884116062059 | eBay
Amazon.com: Dell Optiplex 990 Desktop Computer (Intel Quad-Core i7-2600 up to 3.4GHz, 16GB RAM, 2TB HDD, WiFi, VGA, DisplayPort, (Tower w/ HDMI), Windows 10 Professional - (Renewed) : Electronics
OptiPlex 990 Ultra Small Form Factor (USFF) Removal Guide for the Coin Cell Battery, Front IO Panel, Control Panel, Intrusion Switch and Power Supply | Dell US
OptiPlex 990 memory upgrade greater than 8GB - Dell Community
Dell Optiplex 990 CPU and Ram Upgrade - YouTube
Solved: Power Supply Upgrade options for Optiplex 990 Mini Tower? - Dell Community