As I mentioned in the ATI vs nVidia thread, I am going to have to recomemnd Rivatuner
For nVidia users, Rivatuner grants an extreme amount of control over your video hardware. You can monitor GPU temperature, monitor and set fan speed, overclock or detune the GPU and shader clocks, and then create performance profiles that can be launched manually or triggered according to some set of conditions. There's a ton of other stuff in there too, but what I listed is what i use it for.
I have a pair of LE 8800GTXs running in SLi, which can run really hot depending on what I am doing. I used Rivatuner to monitor the GPU temperature and fan speed in the windows task bar, and also created a couple of performance profiles that detune the cards and crank the fans up when the temp reaches a certain threshold. Now I run hitch free.
One word of advice: If you update your drivers past the version that Rivatuner supports, your performance profiles may report being invalid and get cleared, though they still seemed to work in my particular case once I had recreated them. now I just wait for Rivatuner updates.