I would just get 4x2 DDR3- 1333 or pc3-10600U desktop only memory which may be cheaper than 2x4 gb similar spec.  Any name brand such as Hynix, micron, Samsung will do.  Used ram on eBay okay as long as seller has tested them. DDR3- 1600 or pc3-12800 is pushing the limit for this model.  Crucial ad is not 100% reliable.  You do not need to get low voltage ddr3 for 780. both regular and low voltage ddr3 work, but I have always used regular voltage type in this model.
Optiplex 780 desktop ram upgrade Problem The PC arrived with 3Gb of ram. A 2Gb and a 1Gb. I think they are 1066. I thought I'd upgrade to 8Gb with two 4Gb crucial DDR3L 1.35v 1600Mhz memory sticks. The crucial memory adviser on their site indicated these would work in a 780. I took out the 1 and 2Gb sticks and put the 4Gb sticks in exactly the same sockets as the old ram. Unfortunately they don't work. On boot up there is no video signal. The PC power supply and fans are making a noise but nothing is showing on screen. A number 3 and 4 shows on the front of the PC under the USB sockets, so I presume there's an error of some sort. I've tried them in different slots. I've reset the bios by removing and replacing the cmos battery, I also put a new battery in. I tried one 4Gb with the 2Gb stick but it won't boot properly. I've looked in the bios to see if there's something I need to change but I haven't spotted anything yet. The system does run with 3Gb of memory running Linux Mint but 8Gb would be better. If anyone can throw light on what might be happening or suggest a solution I' appreciate it.   Regards Steve    
Optiplex 780 Desktop ram upgrade