Without knowing the exact model of the CPU chip, the maximum temperature for Pentium D chip ranges from 63 to 69°C. Anything below that is good, 40-50°C would be ideal.
That is a really tricky question because you could have a different answer depending on the the climate you live in i.e. hotter climate the cpu will run warmer and colder cooler... normal range is between 40c & 50c it should never go over 80c.
A good temperature should run in the case of 25 Celsius to 40 Celsius. Anything hotter runs the risk of damaging the cpu. If your running hotter replace the fan.