Just set the laptop for "Max Battery" power mode and it will dim the screen and drop the processor speed. Make sure it's an Intel Centrio not a Celron to minize power consumption. I always elevate my laptop with something under it so it doesn't heat up as much... two pencils, one of those laptop coolers.
Watch out the new Intel Macbooks run really really hot...
Watch out the new Intel Macbooks run really really hot...