Heya Zelda,
ATI tool is a great little program. If you let ATI tool do the auto clock, it will keep overclocking until it sees artifacts. Once it does, then it dials the speed back I believe by 5%. You also want to keep an eye on the temperature to make sure it can run at a stable temperature over time. 75-80 degrees celcius under full load is about as high as you'd want to get.
Keep in mind, going for a max overclock is going to wear your video card out much faster than normal.
Hope that helps!
