I don't know all that much about how the RAMs work on the inside, but I do know that the xbox have got an EEPROM. It's usually called "the EEPROM". Every xbox have one, and on the 1.6-models there where changes to it.
The system reads config from EEPROM on boot. It's very possible that the mem-config is written to it at the same time as the serial, region and HDkey.
Anyway. Even if it is like you say, we should blame it on faulty PSUs, that would be very simple to check. Just switch PSUs between one working 1.6 and one that is crashing.
Of course, that does not explain why Yoshis Beta4 is working on some boxes that crashes on the ordinaly M8_16.