The XP processors have a thermal sensor on the die itself - giving you a much more accurate, and higher temp reading - the nforce and and a few other boards use the same for thier temp readings, while others still use a diode on the back of the chip mounted on the motherboard.
I get the very same readings with a thermal sensor placed right beside the CPU die under my heatsink.
When I turn on a game that maxes out the CPU (or Seti@home) - the temp reading instantly rises to 65-70C, when I exit the game - it drops right back down to 45-50C. And this is with a XP1800 using a HCC-100 heatpiped copper heatsink and good thermal compound.
My motherboard's diode however - reads 40C and only climbs to 43C 2 minutes later when maxed out due to the efficient CPU cooler.
Just remember, your temps are reading higher - but they were always this high, just most of us never knew. This is the most accurate readings you'll get.