I purchased 2x4gb of memory to upgrade my Optiplex 780 which originally came with only 2 gbs. I did some research to make sure the memory I purchase would be compatible with the motherboard. I purchased the following memory from amazon:

www.amazon.com/.../ref=oh_aui_detailpage_o00_s00;psc=1

when I put in the new memory, the PC boots up normally, fans working and all, but I get absolutely no signal/video to my monitor. When I pop in the old 2 gbs, the computer boots up normally and i can use it. 

Can you please advise how to make the new memory work, if i have to go into bios?

thank you in advance.