I can't stress enough to use the Task Manager to monitor your CPU usage. If you don't hit 100% when gaming then you are NOT limited by your CPU and you are creating a more unstable system and reducing the lifespan of your CPU for no benefit.
Games vary. For example, I have an X2-4800+ overclocked by 20% and a stock HD3870 512MB graphics card. In 90% of the games, I am NOT limited by the CPU and it averages close to 80% usage meaning my overclock really isn't affecting these games at all.
In Batman Arkham Asylum and Mass Effect both of the cores run at 100% which means I need a better CPU, but it is also the reason I overclocked otherwise I would not have done so. Both games do run smoother but a slightly better system (and PhysX for Batman) would drastically improve things.
FWIW, Batman AA is a truly awesome game. I'll play it again when i upgrade.