Then it does sound bad. DOTA is not graphically demanding. Even the older 640 should have been able to handle it at max settings without a hiccup.
When a graphics card over heats, it
normally does not cause a total system shut down.
The problem can be attributed to a failing power supply, a hard drive malfunction, or even a corrupted driver.
The first solution is to completely remove your graphics card drivers, turn off the PC, and remove the card itself. Dust it completely and take care to check for any cracks in the PCB or any hemorrhaged coils. (They will look like tiny metal cylinders on the inside of the card near the fan and the heat sink. If they are busted they will appear split with white strands of insulation coming from the sides.)
Once you've determined the card is not at fault, reseat it into the PCIe slot and power the computer back on, then do a clean install of the graphics cards with the latest version of Nvidia.
http://www.guru3d.com/files-details/display-driver-uninstaller-download.html
You can completely remove your drivers with this utility.
This will test and see if the card itself is the problem... If it is not, then we have to move on to another component of the system until you find what the problem is.