Hello.
The powerbooks generally feel hotter because of their aluminum case, which is designed to radiate the heat from the processor to the outside.
Temperature-wise, my fan will kick in when my CPU reaches about 142F. My overall temperature hasn't gone above 149F (when playing Battlefield 1942 or something!) But one thing should be mentioned when comparing the powerbook to a PC laptop....noise.
I can hear my fan when it turns on, but it's relatively quiet. However, the majority of PC laptops sound like wind tunnels.
I would recommend investing about $20 or more in a decent coolpad. There are some out now that have fans that run off of your USB port. My friend has one, and it keeps his powerbook relatively cool.
Hope this helps