Got my additional RAM today, and I confirm everything worked after I installed 2 more DIMMs! The RED letters in the Memory Module Guideline is there to emphasize which DIMM slot they should go in, not they are optional. So with 2 DIMMs per each CPU, in DIMM slot 1 and 2 per the guide, it finally POSTed and I was able to get into Setup, etc.   Summary: For the dual processor config, two DIMMs per CPU must be populated!  
I picked up a used T7810 at a local auction for $100. It came with two E5-2630 V3, NO memory and 825W power supply. I bought this box to build a high-core/high-memory lab server cheap to tinker with. I also eBayed a pair of E5-2683 V4 and also a pair of 32GB RDIMM ECC modules, so I was hoping I'd be installing the OS this weekend. And of course, that didn't happen as planned.   FYI I have zero experience with Dell hardware. So when I plug in the power cord into the PSU, I see the motherboard fans and the PSU LED turns on briefly. Beyond that, nothing happens. The power button at the front doesn't seem to do anything at all, no LED activities whatsoever. The power button does flash white very briefly once, when I plug in the power cord into the PSU. Then I read about the PSU self-test button (!), and I decide to press it. The green PSU LED remains on while I'm pushing the button, gets all the CPU/MB fans going at FULL SPEED (I was quite alarmed!), but doesn't seem to actually get the system to "power on". There's absolutely no video, or front power button LED, or beeps. I tried swapping the CPUs with my "new" E5-2683 V4s, but it behaves exactly the same. I also tried pulling out the 2nd CPU and moving the DIMM to CPU1 DIMM2, but nothing changes. (I wasn't certain whether I can run two 32GB modules, one each in each CPU's DIMM#1 slot; the Dell manual didn't specify I can run DIMMs in single channel?) Also replaced the CMOS battery, but no changes. So, even though the PSU self-test seems to be passing according to what I read (green LED?), I'm convinced that it's the PSU. I mean, it's either that or the motherboard IMO. I just ordered a used 685W on eBay for $25, which will be here in a week. Love to hear your thoughts!  
Bought a used T7810 and it won't power on - please help!