1.) You don't know as much as you think you know. PC133 and PC100 are the same thing, structurally. PC133 is simply validated to run at a higher speed. There is no minimum speed. Like the last guy said, mix them all, and run them all at 100MHz. You should put the slowest DIMM in the first slot so that BIOS will use the slowest SPD timing.
2.) Yes, as long as the bus speed is set to 100MHz or the board supports split settings (FSB133/RAM100).
The ONLY problem you'll EVER see with running newer SDRAM memory on older boards is with density, read the FAQ about that.
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