390, maximum memory?

Yes.

Yet another working Optiplex 390 MiniTower setup with 16GB of RAM.
I used the 16GB Corsair 8 GB DIMM 1600 MHz PC3-12800 DDR3 Memory (CMZ16GX3M2A1600C9).
Yes, that's an i7-2600 SR00B in there!
Below, from Piriform's Speccy:

Operating System
Windows 10 Pro 64-bit

CPU
Intel Core i7 2600 @ 3.40GHz 29 °C
Sandy Bridge 32nm Technology

RAM
16.0GB Dual-Channel DDR3 @ 665MHz (9-9-9-24)

Motherboard
Dell Inc. 0M5DCD (CPU 1)

Graphics
Acer G276HL (1920x1080@60Hz)
1024MB ATI AMD Radeon HD 6670 (Dell) 31 °C

Storage
465GB Seagate ST95005620AS ATA Device (SATA ) 34 °C

All appears to be going well, but it's only the first day with 16GB.

If this shows up three times, I apologize, as this post does NOT show up.

I just wanted to confirm that i'm using also 16GB of RAM and the i7-2600 SR00B in my optiplex 390 running with BIOS A14

Today I got 1 8gig stick by mistake. I had 2 2g sticks and wanted to increase to 8gig. Now I have 10gig - one 8 and one 2. I will get another 8 soon, but the 8gig limitation is not true for i5 with 11 bios.

I made mistake. My BIOS is A14

FWIW, the technical guidebook for the 390 does not list a RAM voltage rating in the supported configurations.

390 unlike the 990 only has 2 ram slots.

4 gigs is max dimm size so 8 gigs is max ram based on the number of slots.

Some time this week, I plan on cramming an Adata 8 GB dimm in my optiplex 790 just for grins.

It will have the 3 neighboring slots occupied by two 4 GB and one 2 GB. All Hynix brand. I wonder if it matters which slots to put them in?

Probably wont work, but seems like folks are getting these dells to run lots of things they aren't supposed to, so I'm going to try.

Well, it worked just fine

So I now have three different sizes, two different manufacturers, and two different ranks, all working in the four memory slots.

They all seem to be running at timing 9-9-9-24 according to hwinfo64. It (and other system info programs) also show dual channel operation, but I know there is 6 GB of memory hanging out without a mating segment of memory in the other channel, so it must be accessing this segment as single channel.

I'm beginning to think all these memory rules are rubbish, but I guess the snake will bite me someday.

Just confirming that I got 16 GB RAM working in 2 OptiPlex 390 SFFs. RAM I used is the G.SKILL F3-1600C11D-16GNT with the latest BIOS update.

Max RAM is always the lowest of the amount the BIOS supports (which you won't know without trying), the amount the CPU supports, and what's actually available for that number and type of slots. For example, the 390's CPU actually supports 32 GB max RAM, but the SFF model (which I have) has only 2 slots and 16 GB sticks of that RAM type aren't available, so I'm limited to 16 GB.

I've got an EVGA GTX 1050 in mine right now. You'll need a graphics card that does not require external power because the stock power supply doesn't have a PCIE connector.

Did you just swap out the CPU?

What bios are you running?

Actually you cant because

KHX1600C9D3K2/8GX

8GB Kit aka (4GB 512M x 64-Bit x 2 pcs.)

https://www.kingston.com/datasheets/khx1600c9d3k2_8gx.pdf

does 16gb work