Ok, let's help you understand this:
PC3200 is "rated" for 400mhz operation. (200FSB at double-data-rate speed is 400mhz effective)
PC4000 is "rated" for 500mhz operation. (250FSB at double-data-rate speed is 500mhz effective)
However, simply installing "faster" ram isn't really what you're doing. The ram is rated to go faster in systems where it needs to, but if your computer is only running your memory at 333mhz, then no matter what ram you drop in, it's only going to do 333mhz.
As such, the only time it really becomes necessary to buy higher speed-rated ram is if you expect to force your system to run the ram at that higher speed. You typically do this by overclocking...
Make more sense?