I try not to overclock my laptop but I have extensive experiencing overclocking my desktop. In the past I overclocked a mobile ATi 9600 on a laptop and gained significant performance improvements without too much of an increase in temps.
One thing to consider is that as you increase the temperature of electronic components you will decrease their lifespan. Typically the default settings will give you temperatures that will cause no problems over very long periods of time under sustained use but increasing those temperatures slightly may start to slowly cause permanant damage your hardware.
Undervolting is a way to decrease temperatures if you can maintain stability. You don't have many options for increasing cooling so you really want to carefully monitor temperatures.
I have personally caused damage to my current desktop graphics card by overclocking it (see my sig below) and running it under heavy load for the past year or so. I see artifacts in some games and 3DMark05 is really bad now even when speeds are returned to default settings. I accept this as a normal part of my use and my own decision since I am about ready to upgrade the thing anyway but damaging the graphics card in a laptop is a much more serious problem.
Just some thoughts to keep in mind. Otherwise, good luck and nice overclock.