Memory upgrade

Thank you for that info Rockstars. That is more than the PC cost me ( £130 for 16GB) £108 for the PC !! Perhaps I'd be better off doing it 8Gb at a time? Naff.

Mixing memory modules is very iffy.  Best to get all the same memory when upgrading.  Also Dell's have a history of compatibility with non-Dell branded memory.  Crucial memory is guaranteed to work in a Dell and what most of the techies on here recommend for non-Dell memory.  If you want to buy some other brand, buy it where they have a liberal return policy in case it does not work. 

Here is the link to the Crucial listing for a 780 Desktop version (there are other versions, such as mini tower, small form factor, etc. and the memory capacity may be different for the other versions). 

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-780-desktop

Consider the age of your model (it's from 2011)  8gb of ram would be smarter--or even just the 4gb you have. Your old model would not be handling any apps that need 16 gb. Buying used ram on Ebay can be risky. 

Also it is not approved for running Windows 10. There are no Dell win 10 drivers for the 780. You must use windows generic drivers. Windows 10 can run fine with the generic drivers in most cases. Dell drivers are available for XP to Win 7. More info here including service manual:

https://www.dell.com/support/home/us/en/19/product-support/product/optiplex-780/drivers

Good luck.  As I noted its iffy.  May work OK and may not.

I've just emailed another Ebay seller to find out if their memory is compatible/returnable . If it is and I don't return it I'll post the details here. Naff