It depends a lot on the mobo and to some extent on the quality of the RAM
Personally I'd go for it - I have 640M of 133 RAM in a board that only supports 100 (because I upgraded other machines) and that box only runs linux which supposedly taxes your system harder than windows (and therefore tends to be fussier about the system it's installed in) - that box has been up and running for several months without a hitch now
Note that 100 SDRAM in a board supporting 133 would be a disaster
In my experience, I've found that single-sided PC133 works fine in a PC100 board, but double-sided PC133 did not (not sure, but I think it's a standard density vs. high density thing).
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