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 64-bit CPU, we advise waiting 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 64-bit 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 32-bit version of XP, and these days more popular software either comes in a 64-bit version or works with the new OS.

Caveats abound, however: Compatibility with existing hardware remains one trouble spot, and - more alarmingly - 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 March 31, 2003. But when you upgrade, you can no longer boot back into your 32-bit OS. Consider downloading a trial version of XP X64 from Microsoft to make sure everything works properly before you switch for real.

More software

Microsoft has made headway with recalcitrant installers that plagued prerelease versions of X64, but many 32-bit apps - including some Microsoft products such as the popular PowerToys for Windows XP - still balk at installing or running on XP X64.

One bright spot: 64-bit compatible antivirus packages are starting to appear. The 32-bit Symantec AntiVirus 10 Corporate Edition, for example, works just fine in XP X64. The first applications written specifically for 64-bit systems will be for specialized professional markets, such as digital content creation and software development. Another area where XP X64 is currently ahead of the game, so to speak, is gaming. Several 64-bit versions of popular game titles have already been released or are on the way, such as Atari's Shadow Ops Red Mercury, Epic Games' Unreal Tournament 2004, and Ubisoft's Far Cry. Eventually these and other 64-bit games will speed game play by completely loading large maps into memory and will offer more-detailed 3D environments.

Hardware holes

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. Such consumer goods as MP3 players (including the iPod) and some scanners lack drivers; graphics cards and various mass storage devices are better supported.

Our advice

Hold off on XP X64 but consider 64-bit hardware --especially PCs that can address 8GB or more of RAM - when it comes time to buy your next system. That way, you can adopt XP X64 - or more likely a 64-bit Longhorn OS - down the road.