check for compatible RAM at crucial.com

Either will work, but at the same speed you now see (faster RAM will simply downclock).  For best performance, RAM should be installed in identical pairs of modules.

You can generally install a DDR4 8GB RAM module with a speed of (2666MHz) in your Dell Inspiron 15 5570. Ensure compatibility by checking your laptop's manual or the manufacturer's website, confirming support for DDR4 RAM and the 2666MHz speed. If your laptop currently has a 4GB RAM module, adding an 8GB module will result in a total of 12GB of RAM, potentially improving overall system performance, especially for memory-intensive tasks. Keep in mind that the new RAM may operate at the lower speed (2400MHz) to match the existing RAM, known as downclocking. It's advisable to use RAM modules with matching specifications for optimal performance. If you have only one RAM slot filled, add the 8GB module to the empty slot. If both slots are occupied, consider replacing the existing 4GB module with the new 8GB module. Always double-check compatibility with the laptop's documentation or the manufacturer's support before making a purchase, and opt for reputable sources to ensure reliability.

Yes, you can install an 8GB DDR4 2666MHz RAM on your Inspiron 15 5570. However, keep in mind that your system's motherboard is designed to handle RAM at 2400MHz by default. If you install a 2666MHz RAM, it will likely downclock to match the 2400MHz speed, so you won’t see the full performance of the higher MHz. This won’t negatively affect your system, but you won’t be able to utilize the RAM's full speed.

If you’re looking for a performance boost, it’s a good idea to upgrade both RAM sticks to 8GB DDR4 2400MHz or go for matching 2666MHz RAM sticks. Just make sure both sticks have the same speed for optimal performance.

Also, if you’re upgrading for overall system performance, consider adding a fast SSD like the Samsung 970 Evo Plus for significantly improved boot times and file transfer speeds. This will complement your RAM upgrade and provide a noticeable speed boost.