I know that it will be worse than your PowerBook. However, the 17" has more room to breathe than the 15"s do. But they still get pretty hot, its not really a laptop anymore. However these problems are worse when you are doing heavy CPU intesive tasks, which is usually done on a desk plugged in.
Additionally their are programs out now that will raise the idle fan speed so that the computer will stay cooler at the price of a little noise.
However, this is the end of the line for me cause I have a MBP CD.