8600 is a bad solution. According to wikipedia, at least, the performance per watt is much worse. What’s needed is a card that packs as many transistors as the 8800 but runs at lower voltage.
Anyway, I think what you’re looking for, cmorrison, is the overclocking tool NiBiTor. It creates custom BIOS images and lets you set voltages not just frequencies. The voltage regulator on my 8600 is capable of four modes. Two of those are currently labeled as 1.2 and 1.32 (8600 is still new and the labels haven’t fully gotten updated yet. On a 8800 all modes are prob labeled. If not, then just take out a multimeter.)
Basically, video cards switch their voltages depending on their work. In 2D the card downclocks, in 3D it goes up. I’m not sure what mode CUDA uses, but it’s almost certainly the 3D one. A tool like NiBiTor can let you use the 2D (or lower?) voltage and probably greatly increase performance/watt. You’ll have to adjust frequency as well, of course.
Underclocking should not permanently damage your card, but it could become a pain in the ass reflashing the BIOS and also reliability testing.
To be honest, though, I haven’t tried this myself. I’d love to hear what kind of power consumption improvements this can yield.