The laptop has, for many people, become the only home computer needed. Fast, yet portable, they offer the best of both worlds, and can be had for $500 (or less). Their lure is understandable. Yet a laptop is also more complex and, compared to a desktop, far less flexible. A desktop user unhappy with a monitor, keyboard or mouse can simply replace it, but laptop buyers have no options. This means picking the right system is both more important, yet also more difficult. Heres what to look out for if youre ready to open your wallet.