I've been seeing some pretty incorrect advice here...
Speaking from first hand experience, I can at least confirm that standard crucial pc133 works in G4's, and not just CAS 2.
My friend just recently bought him a shiny new dual G4, and I just told him to buy from crucial. The silly old boy bought 2x256 meg sticks of CAS 3, non ECC, unbuffered PC133. We plugged it into his 2 open DIMM slots, hit the power switch, and it booted up just fine. In "About this Macintosh", it had him listed at the full 640 megs of RAM we had installed.
I see no reason crucial's pc100 won't work in a mac with a 100mhz FSB, so I wouldn't worry dude.
EDIT: The silly part is referring to hs getting CAS 3, not the fact that it was also non ECC and unregistered. I realized this might be a little confusing, for what it's worth.