My overclocked Athlon 800 running at 1Ghz, reaches a maximum of 50 degrees celsius under very heavy load. This is an acceptable temperature. It also depends on the cooler you are using, thermal paste, case cooling etc..
If you put your processor under heavy load for a long time, the temp will rise slowly, but it will even out at a certain point, in my case, that's after 10 minutes or so.