To be pedantic, it needs a 64K machine like the 1982 1200Xl or 1983 800XL so would not run on the (max w/o 3rd party mods) 48K Atari 800 from 1979. But yes, wow!
That's for the Atari parts using linear addresses. There were numerous third-party memory expansions that used bank switching as in the XL's to get much more. I wrote Point-of-Sale software using 128K banked expansion for an 800.
I also had my 400 upgraded to 48K with a mechanical keyboard.
Yeah, the 16KB cards were from Atari and the banked switched cards were from third parties. Antic Magazine (named for the chip) had a lot of ads in the back for those.
Yeah, you had ROM space (and potentially cartridge ROM space) eating into your 64K on all the models and had to bank-switch for anything more than 48K IIRC. With a BASIC cartridge in place you'd lose another 8K.
Also I don't recall any of the 400/800 3rd party upgrades being compatible - they used a different bank-switching mechanism - with the later XL/XE models.
"bank switching" sounds familiar, so I'm pretty sure my memory of 64k is correct then. I can't say I understand what it means, but I remember something of that term at the time.
You group your memory into groups of 16k (could be other sizes, 16k is what I recall but it has been years). Then you have a switch that you can program the switches between groups (also called banks). You can only access one at a time, but you can switch between them very fast. It was possible to get up to 5 MB of memory that way, even though you could only use 64K at a time.
Where did you get a real keyboard for the 400. I searched and searched and could not find one. I was so envious of the Timex / Sinclair folks who had a nice replacement.