Usually getting a kit from Crucial.com is a good move. They sell kits for each system, quality is good, lifetime warranty, and guaranteed compatability. This can be an issue with old Dells. Since that has 4 RAM slots 2more2GB DDR3 1333 https://www.crucial.com/compatible-upgrade-for/dell/optiplex-990-small-form-factor#memory
This is "low density" x64 RAM. newer x128 "high density" can be cheaper but won't work.
If you're going to put a 4GB GPU like a GTX1050TI in there I would bump up to 12GB by adding an 8GB kit.
Usually getting a kit from Crucial.com is a good move. They sell kits for each system, quality is good, lifetime warranty, and guaranteed compatability. This can be an issue with old Dells. Since that has 4 RAM slots 2more2GB DDR3 1333 https://www.crucial.com/compatible-upgrade-for/dell/optiplex-990-small-form-factor#memory
This is "low density" x64 RAM. newer x128 "high density" can be cheaper but won't work.
If you're going to put a 4GB GPU like a GTX1050TI in there I would bump up to 12GB by adding an 8GB kit.