There were three versions of XP released as well as the two subsequent service packs; XP Pro, XP Home, and XP Pro 64bit. http://www.microsoft.com/windowsxp/64bit/default.mspx XP Pro and XP Home are strictly 32bit OSs.
The one thing not mentioned was the XP 64bit version's backward to 32bit capability for most programs. Likewise the new 64bit cpus can run 32bit as well as 64bit since they are models now able to run 64bit softwares.
The only version of XP that supports 64-bit is Windows XP Professional x64. It's really not as good as it seems, since there are alot of drivers that dont work with it, as well as some programs. And most games/apps arent 64-bit yet.
When the volume of softwares available are primarily 32bit it will take some time before 64bit becomes the standard just as 32bit did when replacing 16bit. The new releases of Vista will most likely be readily backward compatable to 32bit. It would be a surprize if that wasn't seen. You are looking at close to five years to see 32bit really start to fade away.
There are numbers of 32bit softwares to choose from since 64bit softwares will take some time to fill the store shelves. Even the 64bit version of XP has a backward compatability to 32bit applications. The stock piles of 32bit softwares would have to be cleared out before 64bit takes full control.