It's not suppose to crash, it's supposed to test for the maximum overclock before atifacting, thn lower it and find a "safe ground" for your mm and core overclocks.
You might experience a momentary lack of control/glitch, or some small artifacting, bu it shoul not crash the entire computer. I'd suggest not overclocking wih ATiTool if it harms the PC, or at all really. Check for overheating...