[protomyth]

Both the 400 and 800 had upgrades to 64K. The Atari 400 required some soldering and the 800 was simply a plug-in expansion.

[CWuestefeld]

Not so. Behind the two ROM cartridge slots there were three slots for RAM expansion, each being 16KB.

[karmakaze]

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.

[protomyth]

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.