Here is some information on your computer, it might be different.

https://www.dell.com/support/manuals/en-uk/optiplex-7020-desktop/optiplex7020mt-v1/specifications?guid=guid-501c6863-5540-4daf-b90e-6ea2ca7910ec&lang=en-us

Memory in those systems works best when the memory is installed in pairs of matching size and speed memory.  This allows the memory reads and writes to be interleaved.  That means the system reads bank 1 while the refresh is taking place in the other bank.  This repeats with the opposite stick.  Read bank 2 while the DRAM is being refreshed on the other stick.  Usually all memory does not have to match.  Just each pair has to match,  So if you have 2 empty slots, you can put a matching pair in those slots.

So memory is installed in pairs, with a stick in each bank.  Make sure you use memory that meets the speed requirements for the version of board you have.  Use the service tag number to get the detailed as-built specs of the machine. 

Beware.  Some boards have a maximum amount that you can install.

If you go into the BIOS it should tell you the size and speed of the sticks of memory currently installed.

If you update the BIOS and you have memory that is slower than that which is called for, the updated BIOS may think the memory is bad because on power up it does a quick check to determine the speed and size of the memory installed.