After more than 18 months of delays, the shipping version of Microsoft's Windows XP Professional X64 Edition operating system is finally here. But even if your PC has a 64bit CPU, you might want to wait for Longhorn, Microsoft's next-generation Windows due out in 2006, unless you work with data-intensive programs such as video rendering, 3D animation, or CAD and engineering.

XP X64 can process data in 64bit chunks and address a whopping 128GB of RAM (up from 4GB in today's systems), allowing a dramatic increase in performance by keeping most of the data an app might need in RAM rather than on the slower hard disk.

It looks and acts much like the 32bit version of XP, and these days more popular software either comes in a 64bit version or works with the new OS. Caveats abound, however: compatibility with existing hardware remains one trouble spot; and Microsoft says that installing the new OS will void your PC's warranty. Vendor policies vary on this, so check with your PC's maker before you upgrade.

Microsoft offers a free upgrade for users of XP Pro who purchased XP after 31 March 2003. But when you upgrade, you can no longer boot back into your 32bit OS. Consider downloading a trial version of XP X64 from Microsoft to make sure everything works properly before you switch for real.

Microsoft has made headway with recalcitrant installers that plagued prerelease versions of X64, but many 32bit apps – including some Microsoft products like the popular PowerToys for Windows XP – still balk at installing or running on XP X64. Also, some small features in XP X64 don't work as they do in 32bit XP. For example, X64's Outlook Express can't check spelling.

One bright spot: 64bit-compatible antivirus packages are starting to appear. The 32bit Symantec AntiVirus 10 Corporate Edition, for example, works just fine in XP X64.

Although XP X64 ships with more than 15,000 drivers – a greater number than any previous Windows version did – and even more are available via Windows Update and third-party developers, many drivers aren't available and perhaps never will be. Consumer goods such as MP3 players (including the iPod) and some scanners lack drivers; graphics cards and various mass storage devices are better supported. Check Microsoft's hardware compatibility list.

Microsoft says that the transition to a 64bit desktop OS will take a few years, and that 64bit computing will be mainstream only with Longhorn, so you’re probably better off ignoring XP X64 and looking at 64bit hardware – especially PCs that can address 8GB or more of RAM – when you buy your next system. That way, you can adopt XP X64 – or more likely a 64-bit Longhorn OS – down the road.