I came across the Dell T340 and between the small size, 8x3.5" bays, and a good used deal I found I thought it would be great as a NAS!
I've been running Proxmox and a virtualized Truenas on it, but with 4 sticks of 16GB ECC Ram thought I was maxing it out.
Two years ago ServeTheHome suggested that Dell might one day update the bios to allow over 64GB total - but that hasn't happened yet.
I finally bought a stick of 32GB of unbuffered ECC RAM, so I now have 80GB total I can try (3x16GB + 1x32GB) and...
Good news and bad news -
iDRAC says only 64GB are usable...
But Proxmox detects all 80GB as usable!
I haven't run into any issues yet and look forward to getting my hands on more 32 GB sticks of RAM so I can finally get up to 128GB of RAM running!
Disclaimer: I take no responsibility and make no guarantees. Changing the hardware in your computer is risky and this configuration is not officially supported by Dell. Your milage may vary.
Comparisons: One person tested 128GB on the T140, and someone else (link missing) was able to get 128GB running on another version but also saw iDRAC reporting 64GB supported.
Update: I've bought three more 32GB sticks of RAM so now have 128GB of RAM total on my T340. Proxmox recognized them all immediately, so I'm excited to say that the Dell Poweredge T340 supports 128GB of RAM (in my experience).
Note, the T340 only accepts Unbuffered DDR4 ECC RAM - which is less common and more expensive than Registered DR4 ECC RAM.