What size/form factor is your 780? [IMAGE] The RAM you linked is not compatible with the 780.  Max per DIMM slot is 4GB, not 8GB.  Please see screengrab from the manual for the SFF: [IMAGE] I also attached the 780 Tech Guide below.
Hello guys! So, I have a Dell Optiplex 780 and decided to make an RAM upgrade. I searched online info about the maximum capacity  and a lot of users all said 16 GB. I bought this kit [URL] (Not from Amazon, I bought it in a store on my country). It's supposed to work, but it's not. I already removed the jumper, reseted de CMOS and everything, but it's still not working. It doesn't even beeps anymore, it just appears two numbers on the machine case de 3&4. The monitor also stays black, it gives no image. The machine was working perfectly fine before I upgrade it.
Dell Optiplex 780 RAM upgrade problems