My pal Patrick Crispen over at the superb Internet Tourbus actually just covered this question in their usual exhaustive detail, and with permission, I'm reprinting their material here too.

Over the next few weeks your favorite media outlets are going to tell you about a new version of Windows XP -- "Windows XP Professional x64 Edition"--that is faster and more efficient, supports up to 128 gigabytes of RAM [250 to 500 times more RAM than most XP users currently have], and even lets you run two single or multi-core processors at the same time.

This new version of Windows is a godsend for anyone who does processor-intensive academic research or corporate database manipulation. Best of all, if you already have Windows XP Professional running on your computer, you can upgrade to Windows XP Professional x64 Edition at no charge.

So, you should rush out and upgrade to this new version of Windows immediately, right? WRONG! What the media ISN'T going to tell you is that unless you routinely use your computer to do complex arithmetic functions with integers between 0 and 18,446,744,073,709,551,615 [the number of pennies under Bill Gates' couch cushion], Windows XP Professional x64 Edition simply isn't meant for you. Worse still, you have to have a special type of computer -- one with a brand new Intel Xeon, Intel Itanium, or AMD Athlon 64 processor running inside of it -- to be able to run this new version of Windows.

How can you tell what type of processor is inside of your computer? Simple!

1. Go to Start --> Run
2. In the box that appears, type sysdm.cpl
3. Press the Enter key on your keyboard

This opens your computer's System Properties control panel. You can also get to the System Properties control panel by pressing both the Windows [flag] key and the Pause/Break key at the same time or by right-clicking on your My Computer icon and choosing Properties. But where's the fun in that?

In the System Properties control panel, the first thing that appears is the General tab. In the top right quadrant of the general tab, find out what operating system you are running. To be able to upgrade to the new version of XP you MUST be running Windows XP Professional. If you see the words "Home Edition," "Windows ME," "Windows 98," "Windows 95," or "Greetings Professor Falken, shall we play a game," anywhere on the general tab, you're out of luck. Windows XP Professional x64 Edition will not run on your computer [although you can still play a nice game of global thermonuclear war on that last example.]

Once you have verified you're running Windows XP Professional, glance at the bottom right quadrant of the General tab and look for the words "Xeon," "Itanium," or "Athlon 64." If you don't see any of these words -- if, instead, you see the words "Pentium," "Celeron," Centrino," Duron," "Athlon," "Boron," "Cesium," "Gluttony," "Sloth," "Avarice," or anything else [including no words at all] -- walk away. There's nothing to see here. Windows XP Professional x64 Edition will *NOT* run on your computer.

If, however, you do have a Xeon, Itanium, or Athlon 64 processor, you're good to go. Your computer is [probably] powerful enough to run Windows XP Professional x64 Edition. Hop on over to this website to exchange your old, 32-bit version of Windows XP Pro for a new copy of Windows XP Professional x64 Edition.

    http://microsoft.productorder.com/clientx64

BUT be aware that 64-bit device drivers are as rare as elephants in Antarctica. Many of your computer's peripherals including your DVD burner, sound card, and home-built PVR interface will probably not work once you upgrade to Windows XP Professional x64 Edition.

So, to sum up, there is a new version of Windows XP specifically designed for processor-intensive academic research and corporate database manipulation. You can upgrade to this new version at no
charge, but:

• You have to be currently running Windows XP Professional.
• You have to have a 64-bit processor like a Xeon, Itanium, or Athlon 64.
• Once you upgrade, your computer's peripherals may no longer Work [at least until manufacturers start making 64-bit device drivers.]

My suggestion: Even if you can upgrade to XP Professional x64, don't. Instead, wait until the next version of Windows (codename Longhorn) is released later this year, um, next year, um, sometime this side of the millennium. Not only will you get the latest operating system, my guess is that a LOT more peripherals will work with Longhorn than will work with XP Professional X64 [other things held equal.]

I hope this helps!