There's a common misconception that laptops are no good for playing games on, but this couldn't be further from the truth. Continuous advancements in mobile graphics technology mean laptops can now push enough polygons to make an Xbox 360 look over its shoulder, and embarrass all but the most extreme desktop PC.
Finding a laptop that's suitable for gaming isn't difficult, but there are some key things to look for when making a buying decision. The first of these, quite obviously, is the laptop's graphics card. Whatever you do, you should avoid anything that uses an 'Intel Extreme Graphics' or 'Intel GMA' adaptor like the plague -- you'll get faster gaming performance drawing the graphics yourself.
Instead, look for a laptop that uses a discrete (non-integrated) adaptor by either Nvidia or ATI. ATI's fastest mobile card, the Mobility Radeon X1800 XT, is a superb performer and will run any game you throw at it without batting an eyelid. Likewise, Nvidia's GeForce Go 7900 GTX card can run rings around most desktop cards, and there are even laptops such as the rockdirect Xtreme SL, which use two of these cards simultaneously in a Serial Link Interface (SLI) for even better performance.
The next major consideration should be the size of your screen, or more importantly, its resolution. Serious gamers, and indeed serious graphics cards love to run games at resolutions close to 1,600x1,200 pixels, as doing so makes graphics look exceptionally sharp. To get high resolutions, you'll need a laptop that has a screen size of at least 17 inches.
Many laptops now come with widescreen displays. These aren't always ideal for playing games, as some older titles weren't designed to run in a 16:9 aspect ratio. As a result, the laptop will usually try to stretch the game image to fit the display (blurring it slightly in the process), or run it in a box with black borders on either side. Most users won't mind this much, but those that do should opt for a laptop with a DVI graphics port which will allow you to output graphics to an external monitor or projector.
As a general rule, you should always buy a laptop that uses as fast a processor as you can afford, as this will reduce the chances of the CPU causing a bottleneck and slowing the graphics card down. Faster CPUs drain battery life quicker, but this isn't a major problem for a gaming machine, as you'll probably use it most when near a main supply.
You'll also want to get a laptop with a fairly large hard drive (100GB and higher if possible), to store lots of games on, and one which has a multi-channel audio system so you can connect surround-sound speakers.