I too have the P4 2.4B processor. I can get a max stable overclock at 3.1ghz with a voltage setting of 1.550 volts, with no stablility issues at all. I would recommend keeping the voltage lower than what you have it set at now, there is a usefull guide somewhere on these boards someone posted on max voltages, I will post the link after I go look for it.
Here it is, orignally posted by NoStyle.