Crucial seems to do a very good job sorting out memory and what is compatible and works well.

www.crucial.com/.../optiplex-780-desktop

Crucial 8GB Kit (2 x 4GB) DDR3L-1600 UDIMM

CT7339271  You will need 2 kits

• DDR3 PC3-12800 • CL=11 • Unbuffered • NON-ECC • DDR3-1600 • 1.35V • 512Meg x 64 •
• upgrade for Dell OptiPlex 780 Desktop system.\