2 useful things to know if you got one of them homebuilt or computer show computers, make sure you have a good heat sink, and also when you install the heat sink there is a soft gel or putty like material you shoul put between the top of the cpu and the fan, it helps to maximize the heat transfer to the heat sink and also make sure you have enuff circulation at the input and exhaust vents on the cpu case itself so your airflow keeps the heat levels stabilized man......the more devices and cards installed also gonna have to be factored.....
here man here you go....operating thresholds depending on the processor....
AMD Athlon (socket) upto 1Ghz
90°C
AMD Athlon (slot) all speeds
70°C
AMD Athlon Thunderbird 1.1Ghz+
95°C
AMD Athlon MP 1.33Ghz+
95°C
AMD Athlon XP 1.33Ghz+
90°C
AMD Athlon XP T-Bred upto 2100+
90°C
AMD Athlon XP T-Bred over 2100+
85°C
AMD Athlon XP Barton
85°C
AMD Athlon 64
70°C
AMD Athlon 64 (Socket 939, 1.4 volts)
65°C
AMD Athlon 64 FX
70°C
AMD Sempron
AMD Sempron (T-bred/Barton core)
90°C
AMD Sempron (Paris core)
70°C
AMD Duron Series
AMD Duron up to 1Ghz
90°C
AMD Duron 1Ghz+
90°C
AMD Duron Applebred
85°C
AMD K6 Series
AMD K6/K6-2/K6-III (All except below)
70°C
AMD K6-2/K6-III (model number ending in X)
65°C
AMD K6-2+/K6-III+
85°C
Intel Pentium III Series
Pentium III Slot 1 500-866Mhz
80°C
Pentium III Slot and socket 933Mhz
75°C
Pentium III Slot 1 1Ghz
60-70°C depending on model
Pentium III Slot 1 1.13Ghz
62°C
Intel Celeron Series
Intel Celeron 266-433Mhz
85°C
Intel Celeron 466-533Mhz
70°C
Intel Celeron 566-600Mhz (Coppermine)
90°C
Intel Celeron 633-667Mhz
82°C
Intel Celeron 700Mhz+
80°C
Intel Pentium II
Pentium II 1st Generation
72-75°C
Pentium II 2nd Generation 266-333
65°C
Pentium II 350-400Mhz
75°C
Pentium II 450Mhz
70°C
*************************P4's intel Link
http://support.intel.com/support/processors/pentium4/sb/CS-007999.htm#Specifications