The following is the conclusion that Tom’s Hardware made when reviewing Windows XP Professional X64 Edition:
Given that there is no difference in performance when running 32 bit applications, it does not make sense to upgrade to Windows XP Professional x64 Edition when running them; you might be upgrading just for the sake of being an early adopter. The general conclusion here is that Microsoft’s WOW64 (Windows on Windows 64) translation layer works well and allows the execution of 32 bit software in Windows X64 without any noticeable difference. This reminds us of the transition from Windows for Workgroups 3.11 to Windows 95: as long as enough RAM was installed, performance was not an issue.
We will spend more time in the coming months evaluating 64 bit applications and possible performance gains, and reporting to you our results.