8GB Kit (4GBx2) Upgrade for a Dell OptiPlex 990 Mini Tower System (DDR3 PC3-12800, Non-ECC, ) at Amazon.com
Refurbished: Dell OptiPlex 990 SFF Desktop PC Computer Intel Core i5 2400 8GB RAM 256GB SSD DVD Windows 10 Professional New Free keyboard,mouse,powercord,WiFi-Adapter - Newegg.com