Most of the Athlon XPs are rated for a maximum die temperature of 90°C, and the temperature probes are most often on the motherboard itself (so they’re not measuring actual die temperature.) Measurement accuracy vary so wildly from system to system that I consider anything over about 55°C “way too hot.” I don’t have temperature figures handy for other CPUs, but they shouldn’t be too difficult to find.
Part of the accuracy problem is that the sensor chips commonly in use report a raw number that needs to be adjusted into something useful. For example, the chip used on my motherboard (Winbond W83697HF) has an ADC that isn’t capable of dealing with voltages over 4.096V, so the +12V, -12V, and -5V pins need to have resistors tied to them, plus there’s a resistor on the reference pin for the temperature inputs. Assuming your motherboard manufacturer used the recommended resistor values for everything, you’re fortunate–you only have to deal with any intrinsic error in the thermistor and any error caused by placement (e.g. the probe-under-the-cpu style which seems to be common isn’t going to get you the die temperature, although it should give you a rough idea.) If your motherboard manufacturer used different resistor values and whatever you’re using to read the sensor data doesn’t account for that, all bets are off. The W83697HF datasheet, linked above, has some of the nitty gritty details (PDF page 26, manual page 22) for the curious.
If your BIOS shows sensor readings, that’s generally a decent benchmark of whether whatever software you’re using to read the sensors after bootup is showing sane values. The values shown have been known to change between BIOS releases, though, so even that may not always be accurate.
FWIW, I have an Athlon XP 2100+ that usually runs at about 35-40°C idle and 45-50°C under full load (assuming it’s around 25°C inside, i.e. my air conditioner is working. :D)