This message was posted on behalf of Intel Corporation
Hello hilbi ,
Thank you for contacting the Intel Communities!
RAM is meant to work together with its "partners" they are required to be in sync to have the best behavior/performance, for that reason, the best option is to have the same model/most similar of RAM to the one you already own in order to avoid any problem.
I found a link that has the recommended "upgrade kits" from the manufacturer of your device, you should check it out: http://www.dell.com/en-us/shop/category/ar/8136
Hope this can clarify your inquiry
If any additional inquiries are present, feel free to contact us back.