Iceman, what he means is this:
PC2400 and PC2700 RAM is rated to go faster than the 133MHz of PC2100 RAM.
PC2100 = 133MHz
PC2400 = 150MHz
PC2700 = 166MHz
Therefore, if you run the faster RAM at a slower 133MHz bus speed, you are essentially underclocking it... Basically, it means you will have room to overclock your FSB and still be within specs of the RAM.
Hope that helps clear it up.