They have 2TB RAM in there, and I suspect that is not the largest possible amount if you're willing to spend more money (though probably the largest possible amount for that particular server).
I played around a bit on the Dell website, and the largest server I could find supported up to 6TB RAM, with a price at ~300k EUR (I assume nobody pays list price for these).
4TB seems to be the limit for normal-ish servers. 32 slots filled with 128GB DIMMS. You can find servers with more slots (48 is common, 6TB), but you're going past most people's definition of "commodity".
I once worked for a company that was writing a proposal for a US Homeland Security IT system. This was 2006. I wasn't involved in it but my office-mate was. He randomly turned his chair around and said "hey, can you go on Dell.com and see if you can build the most expensive server imaginable" – so I did and I ended up at around $350k. I don't remember what it was, but at the time the stats just felt absolutely obscene. I do remember that, at that scale we were buying a closet, not just one box.
And I told him the price, and he said, "They have set aside $20 million for server hardware, and what you configured is way way overkill for what they think they need" – so we were both so damn perplexed because, at least from what he told me, this didn't need that much. And it wasn't one of those classified projects where they couldn't tell you what it was for. It was a pretty boring line of business type thing (like HR or building access control or something).
Maybe that's probably more a story of just how much cash was being poured into that agency during the years after 9/11.
How long does it take to get the data off disk and into memory after coming online? A decade ago filling just 64GB of memory with our hot dataset was a painful process. I can't imagine it's any nicer with 768GB.
On a modern raid of NVME, you can do several gigs per second - with 20 PCIe Gen3 lanes on a 3400G about 10 GB/s is reasonable, for a theoretical max of 20GB/s.
Assuming your system is very badly configured and you can only do 3 gigs per second, about 400 s, so about 5 minutes, which is not very painful
In memory provides real-time transactions you can't guarantee when using disk-based storage.