yes you can although Dell has not validated 8GB stick for 990. [URL]
Hi, I have a dell OptiPlex 990 SFF desktop and I am wondering if I can put four eight gigabyte sticks in it to give it 32gb of ram(I currently have 4 4gb sticks in it).  The reason I am asking is because I have read in other places that you can only install 4gb sticks but I am confused because I can supposedly put up to 32gb of ram in it. Device info: [URL]  
Ram Upgrade for Dell OptiPlex