PC2400 isn't an official standard. It's simply PC2100 DDR SDRAM which is guaranteed to be stable at a FSB of 150 MHz. Just as a side note, PC2100 is for a FSB of 133MHz.
If you're not gonna be overclocking, then PC2100 is perfect for you since you won't be using the extra clock speed the PC2400 will be able to handle.