Here's my way of overclocking with ATI Tool:
Find Max Core, until it stops responding.
Remember the last clock it was at, before it starts freezing. Then just back it down by 10-20MHz, and use the Artifact Scanner for at least 10-30 mins.
Then do the same procedure with the Find Mem.
Then make a new profile, in the upper left hand corner, and name it something like, overclock. Then save the settings for that profile.
Also if you want the overclocked settings to stay, when you boot up your system:
Go to settings, from the drop down menu, select start up, and select the box that says, "Every time ATI Tool is launched, load profile:". Then just select the overclocked profile, from the drop down menu.