AnandTech's preview of Windows XP for AMD64
People at AnandTech have taken a closer look at the performance improvements of 64-bit Windows XP, for 64-bit processors like AMD Athlon 64, over 32-bit Windows XP. They have used Release Candidate 1 of Windows XP Professional x64 Edition (which you can download for free from Microsoft) and found out that even with a pre-release version and not yet optimized drivers it is faster in almost all cases. The only big exception in this are games, which heavily depend on good drivers, especially for graphic cards. And unfortunately nVidia and ATI will have to do a much better job here in the future. But currently they are mostly focusing on stability and not so much on performance, which is understandable. The situation will surely change after the release of final version of Windows XP for AMD64. So currently you can in most cases only gain by upgrading to 64-bit Windows XP, unless you are playing games a lot, of course.
You can read the complete preview here:
Windows XP 64-Bit Preview: First Look at Athlon 64 Performance