Memory is as important on a laptop as it is on a desktop. In fact, because laptop hard drives tend to be slower than their desktop counterparts, it may be more important--after all, the more RAM your PC has, the less often it needs to load data from the hard drive. It's a good idea to buy at least 4GB of RAM for your laptop, if it's offered as an option. Beyond that, the benefits are usually small, and the cost to add more RAM is very high.
Laptop memory comes in two types, DDR2 and DDR3. Of the two, DDR3 is faster and can speed up memory-intensive operations. You'll also see a clock speed listed on some laptop memory specs, like 667MHz, 800MHz, or 1066MHz. The higher the number, the faster the RAM. But spend the money to get to 4GB first, and then worry about speed: If your choice is between 4GB of 800MHz DDR2 memory or 2GB of 1066MHz DDR3 memory, go with the 4GB of slightly slower RAM, as you'll get more performance bang for your buck by doing so.
The size of the display will be determined in part by the type of laptop you buy: By definition, netbooks have smaller displays than desktop replacements. From there, you have several additional factors to consider: screen resolution, LED backlighting, and a glossy or antiglare surface.
Screen resolution is a measure of how many pixels are on the screen, horizontally and vertically. A netbook with a 10-inch screen may offer a resolution of either 1024 by 600 or 1280 by 768, for example; in this case, the screen size is the same, but the latter option will have a lot more pixels crammed into it. This gives you more space on your desktop and lets you see more of the Web pages or spreadsheets you view, for instance. On the other hand, the higher resolution makes all of the icons and text appear smaller, so things can be harder to see. Most users prefer higher resolutions on their displays, but you might want to look at two laptops with the different resolutions you're considering to determine whether you prefer more desktop space or larger icons and text.
Many laptops have LED-backlit displays. Instead of compact fluorescent tubes, LEDs (light-emitting diodes) sit behind the LCD panel. LED-backlit displays tend to be more energy-efficient, so the battery lasts longer, and they often provide better contrast. LED-backlit displays are increasingly common, and now can be found in all laptop segments and on most notebook models, at least as an option.
You'll also notice that some laptops have a very shiny, glossy display, while others have a soft matte finish on their screen. This is a matter of the coating on top of the display. A glossy coating certainly creates a lot more glare, but it also lets light through more easily; as a result, glossy displays tend to look like they have better contrast and brightness. The matte finish on other displays may result in the appearance of a little less contrast, but it also produces a lot less glare. If you plan to use your laptop outdoors or in brightly lit areas, you might want to consider avoiding a glossy display.
Touchscreen displays are starting to appear on some notebooks, from convertible tablet laptops to all-purpose machines and even some netbooks. Finding a portable with a touchscreen is still relatively rare, but it is becoming increasingly common. Expect to spend $100 to $200 extra for this feature.