Windows XP 64-bit final version is currently available for download, for all the MSDN subscribers. The retail version is expected to be available in three weeks.
The new Microsoft OS has been extensively delayed reaching gold status, oddly enough, just as Intel is ready with its 64-bit chips. However delayed, the creation of an OS providing a pure 64-bit environment for such processors should have a significant impact on overall performance and should allow new exciting features to kick-in. Most of those will probably begin appearing in the guise of service packs after developers get some experience and a proper feel for the environment.
AMD currently has 64-bit server, desktop and mobile processors available so it covers the full range of options while Intel, back in February announced the 600 series of Pentium 4 Extreme Edition HT 64-bit processors.
The creation of a proper and stable OS is a lengthy process and although AMD must feel bitter for having a product, essentially, handicapped by the delays, the company will have to understand the intricacies involved.
If you haven't heard that much about 64-bit computing yet, make a note as today is the day when it goes mainstream and since all future processors are destined to be 64-bit only, (unlike current backwards compatible ones) prepare to be outdated. Just as a sign of the way things are heading you should bear in mind that all 3 major next-generation consoles will feature 64-bit processors.