Go to Synaptic/search/ temperature
"Computer Temperature Monitor applet for GNOME
Computer Temperature Monitor is a little applet
for the GNOME desktop that shows the temperature
of your CPU on screen."
You can log temperatures to a file and set alarms as well.
More information: http://computertemp.berlios.de/
If not already installed, right click
"mark for installation" install
Or if you want install Gkrellm, also in Synaptic, and all its plugins and get everything