DO NOT upgrade to WinXP Pro x64 Edition. It breaks a lot of graphics related things. On top of that, there's next to nothing in terms of graphics applications that are compiled for 64-bit operation. Basically you'll take a bunch of risks and get no benefit.
For instance, this 64-bit version can break Adobe Acrobat. Various users on the Adobe forums are screaming for a 64-bit update. It isn't coming anytime particularly soon. 1Q or 2Q 2007 is the likely date -when the rest of Adobe Creative Suite 3 is launched.
WinXP Pro x64 causes issues with other Adobe applications as well.
Do you use any Type 1 Multiple Master fonts by way of Adobe Type Manager 4.1 Lite or Deluxe? You'll lose that capability running 64-bit Windows.
If any of your applications, plug-ins, application installers, device drivers, device driver installers and just about anything else that executes code uses anything compiled in 16-bit words it will not work in 64-bit Windows. There is lots and lots of software and device drivers that fail that test.
When WinXP Pro x64 was released there were big problems amongst anti virus software companies in getting their applications to work.
Others can do what they like, but my plan is this: I won't upgrade to WinXP Pro x64 edition at all. If I have to buy a new desktop PC anytime soon, I'll get one running XP Pro. I won't feel comfortable getting a PC pre-loaded with Windows Vista unless I absolutely know my applications will run on it. If I have to buy a bunch of upgrades to gain compatibility, then that certainly will make me weigh all the options (including getting a Mac and Mac versions of my apps) but definitely make the wait for a new purchase longer.