As there aren't yet any replies to your question, I'll give my - non expert - opinion.
Memory can generally run asynchronously, i.e. it can run a a frequency different from the FSB of the processor. This means that even if your processor FSB is 133M which would normally run PC2100 memory, PC4000 memory can be used and will run faster, however the processor itself will still not be able to use the data faster than it's own clock speed will allow, so there will be little advantage.
Crucial, who make some of the best memory available say this
"DDR memory was designed to be backward compatible so generally speaking, you can safely add faster memory to your computer. For example, you can install a PC3200 DDR module in a computer that calls for PC2700 DDR. However, keep in mind that faster memory will not necessarily make your system faster. You can't speed up your computer by adding faster memory if other components in your computer (your processor or other memory modules) operate at a slower speed.
The right memory for your computer is the kind of memory it was designed to take. Check your system manual or look up your system in the Crucial Memory Advisor to find the memory guaranteed to be 100 percent compatible or your money back." click here
The exception, as I see it to the sentence "The right memory for your computer is the kind of memory it was designed to take." is that as the technology progresses makers just don't make the slower memory so you will probably soon not be able to find the slowest modules such as pc2100.