first look into your motherboard bios and disable spread spectrum in order to overclock properly make small fsb raising by 5 to 10 mhz then test the overcloking by
using prime 95 if its succesful the log entry shall tell you everythings fine,
you should get core temp,
cpu z,
and get to know your pentium d specificated processor .
thats how you will know the limit from the proc specs,