You can't do it, or to be more accurate you can, but it's very difficult and very expensive. So it's really not a good idea.
If you're going for a laptop, I'd recommend at least 1Gb of RAM (2Gb if you're going for Vista), a Core 2 Duo CPU, a 7200rpm HDD, and a 7400 class or better card. The 7400 will handle older games at lower settings without pushing the cost up too much and without sucking all your battery life. A 7600 class will handle games much better at the cost of battery life and mobility, plus the laptop will cost more.
Also consider your screen, it's for life in a laptop. Make sure you get the size and resolution you want. And think about whether you want a glossy or matte screen, if you're not sure go to a local computer shop and ask to see some laptop screens to get a feel for them.
If possible, try to test out the keyboard and touchpad of the laptop you want, as you can't change those either.
Hopefully this gives you a good starting point.
