Notebooks Help with choosing notebook.

Hi everyone. I need help in choosing a reasonable fast notebook which will be competent in games like Left 4 Dead, Killing Floor, Oblivion and Fallout 3 (Fallout not so much).

£600 is my absolute maximum for this but I would ideally like to keep the price between £400 and £500.

Other than the gaming I would use it for web browsing and that's pretty much it.

I would also be happy to buy a cheap laptop and install a graphics card myself, something like the Mobility Radeon 4850 (or whatever graphics card you would recommend).

I would really appreciate any help you can give.

Thanks a lot