Better to go with two memory modules, as you get "dual channel" operation. However, its also best to have two of the same memory modules. Mixing brands can, and often does, result in problems. Here is the Crucial listing for the 660S, what we suggest for non-Dell branded memory.  Dells have a long history of compatibility with some memory and Crucial is guaranteed to work in a Dell. Memory RAM & SSD Upgrades | dell | inspiron desktops | Inspiron 660s | Crucial.com
Hi, My Inspiron 600s has got 4GB RAM and no memory left out even to open two browser tabs with Windows 10 Home edition. Want to upgrade the RAM to 8G. Please suggest suitable RAM to buy. It has two DIMM slots and currently I am using one 4GB RAM in one slot. Can it be replaced it with one 8G RAM or should I go for another 4GB RAM and make it 2x4GB.  Does the mother board support 1x8G RAM in the slot and what is the clock frequency I can go for. Current RAM is 800MHz, though the specifications say that it can take up to 1600 MHz.   Specs: Speed: Up to 1600 MHz Capacities: 2GB or 4GB                 >>>>>> Does this mean I cannot use 1x8GB and only use 2x4GB RAM ? Minimum RAM: 2GB Maximum RAM: 8GB
Inspiron 660s RAM upgrade to 8GB