Just a brief post to report success with running an X399-E board with 2950X CPU and 256GB (8x 32GB) of Corsair Venegance 3200MHz DDR4 RAM installed. My use-case is VMware ESXi so unlike 99% of the posters in this forum who obsess over what vendor/die their RAM chips came from, I only care about RAM capacity and stablity and have no interest in overclocking.
For background: I was already running 128GB (8x 16GB of 3200MHz Corsair Venegance DDR4) purchased from the excellent QuietPC.com but needed to run more VMs for work. All the publiished specs for the X399-E show 128GB max, but 32GB DIMMs didn't exist then. After a lot of Googling I was able to find a few success reports of people using newer 32GB DIMMs on the X399-E, so took a gamble and persuaded my boss to let me expense 8x 32GB DIMMS. The board was still runnning a 2018-era BIOS so I updated that to the latest one and installed the DIMMs; 64GB to start, which worked, then 128GB, then 192GB, then 256GB. I had a brief moment of concern with the 192GB configuration as one of the DIMMs didn't show up, but after removing and reseating that DIMM pair properly 256GB is showing and all is good. The motherboard has all the BIOS RAM settings in 'auto' mode and the RAM is downclocked from 3200MHz to 2100MHz, but it was previously at 2600MHz with the 128GB configuration so that wasn't unexpected. Everything runs stable so I'm happy and have no plans to experiment with running it faster.
I'm probably not the normal user here, but hopefully this gives others the confidence to run their X399-E boards with 256GB configs
🙂