Keep in mind that power and thermal management greatly affect laptop computers. For example, a Mobile AMD Athlon 64 bit processor would conserve more power, and manage better than a laptop with just an AMD Athlon 64 bit processor. Same applies with the intel range of processors.
Try and stick with a laptop which is designed for optimum portability, which will conserve power and thermally manage well. That way it will last longer when you are out and about without a power outlet. I suggest you avoid laptops that do not have mobile processors.
Unfortunately, an AMD Mobile 64 bit processor laptop is comparatively expensive. If you're just word processing, surfing the net, instant messaging and that kind of thing, then maybe a 32 bit processor would be cost effective. Like a centrino