The bus should lower itself to the speed of the lowest RAM. In this case PC-100. Now on certain machines, the whole bus is effected. On some machines just the memory bus is affected. I am not sure if the QS will drop the whole system bus, or just the memory bus.

No way in hell does it auto-OC the RAM from PC-100 to PC-133.

Sorry but I've tried moving memory out of PC100 machines into PC 133 machines and they DO NOT WORK EVER in my experience.
What you say also makes no sense because if the system bus drops, so does the speed of the CPU, the drives,and everything else that depends on the bus for it's timings. Putting a PC100 stick in a 533 Digital Audio (which I have never gotten to work anyway) would lower it to a 400mhz machine if your logic follows.

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Exactly. PC-100 does NOT work in PC-133 boards at PC-133 speeds. IME moving the RAM from a PC-100 machine to a PC-133 machine is hit and miss. Either it will work (albeit at slower speed), or it won't run at all. In the rare instance you can find PC-100 RAM that is happy at PC-133 speeds.