If you used PC133 it would work. BTW there is more point to it than you think because if the RAM is still running on a 66MHz FSB you can run PC133 (or PC100) at really fast timings with not a cat in hells chance of any damage to the memory so it would be of some benefit to the system.
Most of the time it will work just fine, but on odd occasions, with some motherboards, you can get compatability problems. Compatability problems can get worse if you try to mix RAM of different speeds (not always, but sometimes).
On newer motherboards, you can adjust the RAM timing independantly of the FSB timing - so you can have, say, a FSB66 procesoor, and 100MHz RAM - look for a setting for RAM speed in the BIOS.
And yes, you right, RAM runs at the speed the motherboard is set to run it at - so if set to 66MHz, that's what it will run at, even if it's PC100 or PC133.
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