No argument on Crucial PC2100. I've not had a problem getting any of several 256MB modules (even in pairs) to 166 2-2-2. Some go well beyond that (albeit with slower timing) and some not much. I have gone a few MHz higher with Corsair modules, but IMHO it isn't worth it.
And, the reality is, the old saying still holds true. It is pick and choose. No module is 100% guaranteed to hit a certain speed and/or timing. You get lucky sometimes, and sometimes you get a "lemon" that only runs reliably at the rated settings.