Hello juliengau,Thank you for your message.
I have had a look in to that part and the OptiPlex 9020. That part will be compatible with that system. The part is an 8GB module and it is the correct speed and type for that system.
The system can support 32GB over 4 memory slots so it can support 4 of those modules. You can see all the compatible upgrades for that system here: http://uk.crucial.com/gbr/en/compatible-upgrade-for//optiplex-9020-%28mini-tower%29
We do also have a great guides and support section on our website here: http://uk.crucial.com/gbr/en/supportIf you do have any other questions at all, please don't hesitate to ask.