PC3200 is rated to run at 200MHz DDR = 400MHz. PC4000 is rated to 250MHz DDR = 500MHz.
Basically PC4000 can run at faster speeds. At stock speeds, PC3200 RAM will suit any Athlon64 or Pentium 4 system. However, PC4000 can provide more headroom for overclocking either of these types of CPU, as it can run faster.
Eg: P4-3.0GHz at stock speeds has a 200MHz (QDR=800MHz) bus. PC3200 RAM runs at 200MHz. Thus the two are quite happy together. Now say you increase the bus to 250MHz, giving a CPU speed of 3.75GHz, and a bus speed of 250MHz (QDR=1000MHz). PC3200 will not run at this speed so you would have to use a 5:4 divider. However PC4000 can run at 250MHz. In this case PC3200 would work, but PC4000 would allow for better overall system performance, as RAM bandwidth would be higher.