|Depends on two things mostly:|
Your room temp and the size of the heatsink and fan on the processor!
Assuming you're in a room like me in the UK, I have a system temp of about 32 degrees and a CPU temp of about 48 degrees. I have the Prescott tho which runs about 10 degrees hotter than a normal P4. I also have arctic silver thermal compound, a 3.4GHz rated Dr.Thermal heatsink, a large front inlet fan and two exhaust fans!
On load, after some heavy duty processing i dont expect to see my CPU temp reach above 58 degrees at most but this is picky, anywhere below 70 isnt dangerous for a prescott.
Hope that helps.