Hmmm. It seems to be the fan or the heatsink on your graphics card that is the problem. The first thing you should do is clean the computer case out - free it from dust. Get a can of compressed air, or in an emergency blow into a small rolled up tube of paper, and get right into the fan and blow away as much dust as possible. That should help a lot.
If that doesn't work, go to a computer store, and buy some thermal paste. Back at home, remove the fan and heatsink from your computer, wipe off all the grey paste-y stuff on the graphics card and the bottom of the heatsink, and put some of the new thermal paste back on the graphics card. The size of your little fingernail should do. Put the heatsink back on and you should be fine.
The power supply has no effect in this instance. There are utilities available that will reduce the clock speed of the GPU. This is the easiest and most effective way to reduce the operating temperature. http://download.guru3d.com/Videocards---…
By the way, is the fan on your graphics card working correctly?