I believe you mean 128 MB (k stands for "kilobyte"

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I suspect people will say that your question to run both 128MB sticks at PC-100 versus only ONE 128MB stick at PC-133 would depend on the TYPE of applications you most frequently use.
If you are heavily into video editing, CAD\CAM or the like, you MAY be better off with 256MB of RAM at PC-100. If you mostly "do" word processing, cruising the "net" and "gaming", using just the 128MB of PC-133 RAM might suffice. This solution might also spare you any RAM compatability problems.