The MOS 6532 RAM-I/O-Timer chip used by the 2600 (and KIM-1 and others) has 128 bytes of RAM. Game cartridges could add 256 bytes more, along with the program ROM.

For an experience today closer to these machines, you can get a $0.10 CH32V003 microcontroller with 2k RAM and 16k flash.

Here's a 1 Euro retro-computer kit using one that has included in that price support for PS/2 keyboard and VGA video output, all included in the 1 Euro -- even the connectors! All from one 8 pin microcontroller chip:

https://www.olimex.com/Products/Retro-Computers/RVPC/open-so...

https://www.youtube.com/watch?v=dfXWs4CJuY0

That's astonishing. They must make a loss on every one they sell. I bet they don't sell many though, they want 21 EUR to send it to Norway!