Desktop or LaptopDeciding on whether to buy a desktop PC or a laptop depends, of course, on how mobile you intend to be. Telecommuters working full-time from a home office can choose between desktop PCs, which generally cost less than laptops and have more upgradeable parts, and "desktop replacement" laptops, which tend to be the most powerful -- but larger and heavier -- of the laptop types. Road warriors, however, on the other end of the spectrum, need mobility and therefore will want to have a laptop; which one to select will depend on finding the right balance between portability and computing power.
Processors (CPU)Although many business tasks, such as word processing, are not processor-intensive, multi-core processors are recommended for professionals because they allow you to run multiple applications at the same time (e.g., Microsoft Word and Firefox and virus scanning software). A dual-core processor will ensure a smoother computing experience; quad-core processors are recommended for graphics-intensive work, heavy database tasks, and other professionals who will be taxing their PCs.
Memory (RAM)In general, the more memory the better, especially if you are running resource-hogging operating systems or programs (such as Windows Vista). I second Mark's recommendation of a minimum of 2 GB of memory. Because memory is relatively inexpensive, though, I think professionals should definitely get the maximum amount of RAM you can buy, as it will give you the most performance bang for your buck.
Business users may need less disk space than consumers who save photos, music, and videos to disk; the exception, of course, is if you're a professional working with multimedia or accessing large files like database files. You can still get an external hard drive for extra space, so a drive around 250GB should do for most business purposes. Get a drive that has a 7200rpm spin rate for faster performance.
Laptop business users should look into getting a solid state drive for better performance and reliability.
CD or DVD DrivesOptical drives are becoming less common in laptops, especially the smallest and lightest ones. While consumers may not need a DVD drive any longer because most applications and files can be downloaded or shared online, a DVD writer is more important for professionals, who may still need to send files on disk to clients or install proprietary software from CD.
Video and Displays
Graphics professionals and those in the gaming industry will want to have a discrete (i.e., dedicated) video card, essential for video and graphics performance. For regular business tasks, however, an integrated video processor (integrated into the motherboard) should be just fine.
If you use a laptop as your main working computer, I highly recommend hooking up an external monitor to your laptop, especially if your laptop has a screen size under 17". The extra desktop real estate can make an enormous difference in productivity.