64bit operating systems and software. Currently there's a 64 bit version of Windows in development - called Longhorn - but this won't be launched for a couple of years at least.
A 32bit machine will work just as well in two or three years time, but it won't run 64bit software. If you plan to keep your computer for more than three or four years you should consider a 64bit machine. Many people tend to replace computers within that time frame, and they'll probably be waiting for the operating system and software to hit the market before investing.