Sorry but with that budget,don't expect great gaming,with this budget you can get laptops with Intel 4500MHD card which is a low end card,so you have to play at low resolutions with all settings turned down in order to play new games like L4D2 and MW2 and also your FPS won't be very good as well
You can probably find an AMD system with the integrated Radeon 4200HD, still won't be awesome for gaming, but it should do a little better than the intel 4500MHD. To game moderately well you'd have to go up to a radeon 4650 or whatever nVidias equivalent(s) are, you probably will not find them in anything under about $800 though.
NVidias eqivalents are the 220/240/260M systems. 4650 is probably equivalent to 240M/9600M level. 9800M and 260M are probably better performers, but the cheapest I've seen them is Asus's $1000 laptops.