Ram Upgrade Dell Optiplex 3020 MT

See compatible ram here to check if the one you have matches-- 

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-3020-mini-tower 

It is better to install memory in matching pairs for best performance.

See the manual here for memory guidelines--

https://topics-cdn.dell.com/pdf/optiplex-3020-desktop_owners-manual_en-us.pdf

It says- To ensure optimal performance of your computer, observe the following general guidelines:

• Memory modules of different sizes can be mixed (for example, 2 GB and 4 GB). But, all populated channels must have identical configurations. Memory modules must be installed beginning with the first socket