ATI catalyst settings are not gaurunteed safe. You should be able to do a small overclock without custom cooling, but if you want to push your card, you should, in some way, improve the ability to remove heat from the card. Whether it be through an aftermarket cooler or increased case ventilation.
Every system (and the room it is in) is different, pay attention to your temps, watch for graphical glitches (known as artifacts) and increase speeds slowly. Understand the thermal limits of your card, and if you happen to run out of room in CCC there are other options available.