This is entirely false because I’ve been working on this OptiPlex 780 USFF with 6GB of DDR3 ram, I highly recommend you to upgrade cause It seriously makes a huge difference in terms of performance and reliability. I only recommend you this if you are not having enough finance to spend on a complete separate setup.

Service Manual says 4 slots on the SFF with a max of 4 gb in each. = 16gb  Open it up to check.

It"s the ultra small form with only 2 slots inside. I was wondering if each slot will accept more than 2GB. 

It's an old computer. Best to just throw it away and save your money for a new computer.

Any Dell older than 6 months should be discarded. Dell has great deals on new and refurb computers that are only a month old. You'll get a computer that's good for 6 months ... maybe even a year or more!

Well I got it for free. I only use it for basic desktop apps and old games. It's not worth it for me to buy a new one when ram is so cheap these days.

My aunt has this USFF desktop machine running under Windows 10. It does not so bad with 8 GB RAM and a Core 2 Duo processor for basic surf, Microsoft Office work and email.I upgraded its original RAM configuration from 2x1 GB to 2x4 GB. Speed is decent under Windows 10 for such an old machine that was bought more than 10 years ago.

RAM specs: 4GB DDR3 PC3-10600 1333MHz. It works with an old BIOS version A7 but it would be a good idea to update to the last version A15.

But it's time to move ten years later! As she likes the USFF form factor, I will buy her a cheap Dell Optiplex 780 USFF with 8 GB RAM, a core-i5 processor and a fast Samsung SSD. All this for about 200 Euros.

usff can do 8 gig max with very specific  ram

https://www.amazon.com/dp/B00AAL6KHG

My mistake: "But it's time to move ten years later! As she likes the USFF form factor, I will buy her a cheap Dell Optiplex 780 USFF with 8 GB RAM". I didn't find a way neither to edit my yesterday's post nor to quote it. By the way, I wanted to say: "But it's time to move ten years later! As she likes the USFF form factor, I will buy her a cheap Dell Optiplex 9020 USFF with 8 GB RAM".

In case someone reported here that he would be interested in boosting such an old Optiplex 780  PC, I would visit my aunt and run CPU-Z utility to grab technical details on these two 4GB RAM sticks that still run smoothly in dual channel mode. I can't even remember the manufacturer, it was en late 2016. I checked that now you can get this 2x4 GB RAM upgrade on eBay for about 40 Euros

Hey everyone, thanks for the replies. I can confirm it worked with 2x4GB korean cards bought from Amazon. I also updated my BIOS and put in an SSD. It works remarkably well for an old machine. The only downside is the graphics card which is not good and can't be upgraded. But I play older games like NFS: Hot Pursuit 2, Counterstrike, and PS1 emulator games. Windows 10 runs perfectly on it. The best part is that this computer was free