They probably need to be low density RAM. This usually means chips on both sides of the module, or x64 in the part#. Cheaper "AMD" RAM won't work. It won't run at 1866, or even 1600 though.
Crucial.com does list a 4GB 1866 module x64 as available for that system. http://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-790-desktop
They probably need to be low density RAM. This usually means chips on both sides of the module, or x64 in the part#. Cheaper "AMD" RAM won't work. It won't run at 1866, or even 1600 though.
Crucial.com does list a 4GB 1866 module x64 as available for that system. http://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-790-desktop