I just purchased an Inspiron 3668 desktop and want to expand the memory. Do I use single or dual rank memory chips, or does it matter
I would check the memory that came with the machine and use whatever rank it is. Based upon information at crucial.com it appears that the Inspiron 3668 only has two memory slots, so unless you are operating a single memory module in single channel mode you cannot expand the memory without removing what you already have. Crucial .com is a good source if you are looking for memory upgrades.
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This system model has 2 UDIMM memory slots & supports a maximum of up to 16GB, DDR4 Unbuffered dual-channel SDRAM @2133 / 2400 MHz speed. We recommend a single ranked memory.