I know that PC100 and PC133 are interchangeable. Obviously 133 is faster. In theory I suppose. I know that a while back when I was building a system I had trouble and ended up swapping out my 133 RAM for my older 100 RAM with no problem. I think, but don't hold me to it, 150 is also interchangeable in the same fashion. It all really has to do with how much bandwidth the motherboard has to transfer data to and from RAM.
I don't know where I heard this, but someone told me 133 was flaky with AMD processors. I haven't seen it to be so thouhg.