Just checked my Temp and its running at 58c, put coolerxp on and it dropped to 54c. I thought mine was a bit hot too but looking on the net my Athlon 2800+ runs pretty hot anyway compared to Intel chips. I even took the case cover off to see if it made much difference, it didnt !
To answer your question, were you doing anything graphics intensive before you checked your temp? or have you overclocked ? my machine goes up another 10c when playing HL2. Applying more heat paste wont cool it anymore, if your still worried you will have to upgrade your cooling options.