would this mean that the new RAM was then running at 2,400 MHz.  Yes. When using RAM modules of varying speeds, all the RAM will downclock to the speed of the slowest module. Would running the new RAM at reduced speed then outweigh the benefit for having more capacity? If so should I just install the new RAM and not the existing? Replacing the existing RAM with the new larger and faster modules would give you 32 GB RAM. Adding the new modules to the existing would give you 48 GB. Unless you routinely render huge CAD drawings or edit high-res video or huge photo/graphics files, you may not see much incremental benefit of 48 GB over 32 GB. Plus, as you surmised, if you use 48 GB it will all run at 2400 MHz instead of 2667 MHz. While 2667 may not be a particularly noticeable increase over 2400, you simply may not need 48 GB. You'll have to decide, but at least now you know the result. 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. 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.
Hi guys I have a Dell Precision 7520 with 2x 8 GB RAM DDR4 2,400 MHz which I'm thinking of upgrading with 2x 16 GB DDR 3,200 MHz.  Reading the owner's manual it says the laptop supports up to 32 GB 2,667 MHz SuperSpeed memory.  I was thinking about putting the existing RAM in the secondary slots and the new RAM in the primary but would this mean that the new RAM was then running at 2,400 MHz.  Would running the new RAM at reduced speed then outweigh the benefit for having more capacity? If so should I just install the new RAM and not the existing? Thanks Nick 
Precision 7520 RAM 2,400 MHz vs 3,200 MHz