That depends. Dell's specs say:"Only systems configured with a discrete graphics card will support 3200 Mhz memory."So, if you have only integrated graphics, then you can use 2666 MHz RAM. If you have only integrated graphics, the 3200 MHz stick may work but downclock to 2666 MHz. I'm unclear what you intend to do. You may already have two 4 GB sticks installed for a total of 8 GB. Do you intend to remove one stick, install the 16 GB stick so that you will use one 4 GB stick + one 16 GB stick for a total of 20 GB? That may work, but a matched pair of two identical sticks would be better as it would run in dual-channel mode. One 4 GB stick + one 16 GB stick would allow only 8 GB to run in dual-channel mode, while the remaining 12 GB would run in slower single-channel mode. Sometimes mismatched RAM won't work together at all. Crucial shows compatible RAM for your laptop. They guarantee it to work, and will refund your money if it doesn't work. Follow the disassembly and reassembly steps in Dell's Service Manual. Look on YouTube for videos showing disassembly of your model. But, be cautious with them, as some use steps or methods that are not appropriate. Use the videos to add to your understanding of what to do, but follow the steps in Dell's manual. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. I know many people do such work without this, but it's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
Just bought a new laptop. Dell vostro 3500 with core i7-1165G7, intel iris and 8gb ram 2666mhz. Now a ordered a single stick of 16gb ram of 3200mhz. Just wanted to know will it work fine or not?    
Want to know will ram work