The audio processing power required for digital DJing can be very high, especially if you mix more than two tracks together or apply audio effects to your mix. At the very minimum, your laptop should meet the system requirements for the DJ software you plan to use. The faster the laptop's processing speed is, and the more RAM it has, the more smoothly the machine can process audio. Look for a laptop with at least a 2.4 GHz processor that has 2 GB of RAM at the very minimum.
Good sound quality is, of course, essential to a DJ set. The onboard sound card included on nearly all laptops is not likely to be powerful enough for DJing. When you're putting your DJ system together, add an external sound card to increase the setup's sound quality and free up system resources for the rest of the computer. External sound cards connect to your laptop through its USB port or slide into the laptop's PCMCIA slot. Many hardware DJ controllers that you can connect to your laptop include built-in sound cards.
Loading a large amount of music into your DJ software can strain the hard drive on a laptop. If the hard drive overloads, you might experience audio skipping or other glitches; in extreme cases, hard drive overload can bring your DJ set to a screeching halt. Look for a laptop with the largest hard drive and the highest RPM that you can afford; for the highest performance level, replace the laptop's built-in hard drive with a solid-state drive. Solid state drives are, essentially, giant internal flash drives: they have no moving parts, and provide extremely fast transfer speeds.
Playing DJ gigs usually involves carting your laptop across town, around the country or even around the world, and the environments that you'll be DJing in are often hazardous to a laptop. Drinks spill on laptop keyboards; sound vibration damages hard drives; and one over-energetic move can knock a laptop off the table and crack the screen. Look for a well-built, durable laptop; for extra protection, look for a laptop for which external cases and keyboard protectors are available.