I prefer
SMC Fan control.
Firstly, activating the Dashboard can increase the temperature straight away, depending on how complex your other widgets are and what they do.
Second, smcFanControl sits in the menubar, providing a constant temperature reading. At the same time it gives you a reading on your current fan speed.
Third, you can set different fan speed settings. I override my "default" fan settings, setting the RPM higher than Apple's default, to keep the CPU temperature down slightly (though possibly at the cost of battery life). I can even tell the fans to go full blast even if it's not necessary.
Macbook 2 GHz C2D Mac OS X (10.4.8) 1 GB RAM