Gaming is all about having good FPS (frames per second) about a minimum of 30 fps are recommended for each game.
So for example, for £400 you will be able to get a laptop that would run a game like "call of duy 4 - modern warfare" on high in game grahics settings.
Where as a game like battlefield 3 might even struggle to play on low settings, depending on what the GPU is.
its going to be difficult to play any graphic intense game like Bf3 or batman, but star wars republic shouldnt be too hard to play on low settings. check the minimum requirements for the game. If your looking for one with gaming abilty then you might have to opt for a cheaper brand like acer, their build quality wont be great but they might just have the necessary components. you could get a decent pc for £400 to play games
Just to share, i use Toshiba for all my gaming because it's the most affordable and most durable. and when i play...i tend to be... violent on the laptop. =)
Mac for graphics and video.
Dell for all docs and internet browsing.