cheapest option is to build a gaming desktop from components and buy a separate non-gaming laptop. the total cost will be cheaper than a gaming laptop.
what is your total budget? It's been reported on Anandtech that Asus will drop a new K53-3D model with NVIDIA 560m - haven't been able to find anything about it on the Asus website, but if it has 1080p display and is a decent price, seems like it would be pretty sick because it is relatively thin and portable.
if you have to go with laptop for mobility then I would make sure about the following criteria:
- Display resolution no lower than 1600x900. 1920x1080 display is ideal.
- Minimum level of GPU needed to deliver decent frame rates for recent games at 1920x1080 resolution is 1GB NVIDIA 460m or AMD 5830m. Get one with video card at least equal or better than this.
- try to get quad core Sandy Bridge CPU if possible
If you are considering any laptop with display of 1600x900, you might consider dropping your video card requirements down a step or two from the 460m, to the NVIDIA 555m, or something similar. Personally I would not recommend any gaming on a 1366x768 display or worse, simply because you would be missing out on a huge part of the visual experience of recent games.
You might want to take a look at some of these Sager laptops. Not only are they generally a much better value than brands like Alienware, Asus, etc., but they are designed and assembled with upgrade potential in mind.
The NP8150 line has just been refreshed with the brand new NVIDIA 560m
base price for 15" http://www.xoticpc.com/sager-np8150-clevo-p150hm-p-2972.html?wconfigure=yes is $1279 as assembled by xoticpc.com
base price for 17" http://www.xoticpc.com/sager-np8170-clevo-p170hm-p-2971.html?wconfigure=yes is $1268 as assembled by xoticpc.com