[andrewas]

There are some power management features, but I dont think you can turn off the graphics card. It goes into Standby mode when the monitor turns off anyway, nothing to display, no calculations to be done.

Besides, your attacking the symptom, not the problem. You need to improve cooling in your case. Extra fans would be one way, improving airflow (round ata-80 cables, tieing other cables out of the way, clearing vents etc) would also help.

If its bad enough that you need more than that, you could consider liquid cooling which moves heat faster, but is expensive. And be very careful installing it, one leak and you fry everything.