Try the Radeon Tweaker
Link. Upon installing it will ask for another program called GTK+ it will automatically direct you to the site for it.
rage3d.com is a good site to discuss ATI vid cards.
Just pump up the core/mem- don't choose asynchronous for the Radeon- at small increments and run some benchmark for it like 3dmark, game demos and check for artifacts and see if it crash or hang up.
There is a option in the overclocking page of the Radeon tweaker that say 'load these settings when windows start' You might want to find the highest stable setting before you enabling that option. If you don't enable that option, the overclocking will reset after restart windows.