Fan Speed and CPU/GPU Temperature

I notice that while my CPU and GPU temperature vary as a function of what I am doing on the Mac, my fan speeds never really seem to change. They seem to always be 499 RPM or 515 RPM. Since they are part of the Mac's cooling system, I would have expected them to increase revs when the machine got hotter.

Are they staying the same simply because even at their hotest (for example the GPU can get up to 73 F at some times) all the components are still running within their planned limits?

Is there any software out there that would allow me to directly control the speed of those fans? I realize that Mac OS X is auto controlling them for me right now - it would be fun to control them manually now and then. Thanks. Yes, I am an engineer!!

73F is not a bad temp to be at. Now if it was 73C, thats when you want to start worrying depending on the type of graphic card you have. Usually ATI Radeons run a little hotter than Nvidias (at least the current ones).

But if your gpu is running at 73F, that is about room temperature.

Oops! My mistake. It is 73 C, not 73 F. I only WISH it was 73 F!

I have read on various nVidia forums that the nvidia 6600 GPU is good well into the 100 C area, so I am not too worried about frying the chip any more, just wanted to play with the fans somehow. I am guessing that this is not possible, given the lack of any software suggestions being offered in response to this request, plus my own inability to turn anything up on the web.