Someone tested Crucial's PC2100 and it actually spec'ed at PC2600, so "better memory timings" are only on paper.
Also, I recently purchased a 512MB stick from Crucial and when I added it, my PC immediately became unstable, so I called them and they sent out a new one (which works fine) and even pointed me to an online page where I could print out a free fedex shipping label for returning the defective stick.
So, buying direct from Crucial is well worth paying a couple of dollars more than getting it elsewhere, and this 20% off and free shipping sale makes it quite a good deal, IMHO. (That is assuming you need more RAM now).