I change the clock settings on my 5870 cards directly in the Catalyst Control Center. It can be done in the Overdrive section. There is a test button that will allow you to test clock speeds for stability before you make them active in the Overdrive section.
You may only get 10Mhz to 50Mhz more out of the core clock and maybe 50Mhz more out of the memory clock. Step up slowly maybe 10Mhz to 20Mhz at a time.
Overclocking isn't nessary for the games you are playing. It doesn't hurt to experiment a little. If you want slightly better frame rates without spending any money overclocking is a good way to get it. Good Luck.