Two issues to consider:
1st: The motherboard must support the memory at the speed you are adding. Many to most motherboards have trouble fully supporting the latest, fastest RAM standards...Investigate completely before purchasing that new motherboard. ASUS, for example, will often only support a high speed RAM module in one DIMM socket, while supporting slower RAM in the second and third slot.
2nd: The P4 3.2 GHz CPU will specifically support a RAM speed based on the FEB (Front End Bus) speed of the motherboard chipset. So if the CPU is built for DDR 400 then get DDR 400. The only possible advantage of getting higher speed RAM is if you want to attempt to overclock the CPU Bus speed.
The simple point to be made is that higher speed RAM is not faster in and of itself it simply allows the opportunity to push things up to that point, but is completely dependent on the CPU and motherboard.