Don't worry. 64-bit systems are not new, except in the Windows sense. XP 64-bit may come out this year but before it becomes mainstream and 32-bit disappears, all the current programs will have to be recompiled for 64-bit. There is such a huge base of 32-bit Windows that manufacturers will support it for years. 32-bit Windows will outlast your current computer.
The chief value of 64-bit is memory addressing and that is only really important for large databases. 32-bit can address enough memory for most applications.
Do't worry about Longhorn. It will not appear for several years, its new file system will be even later and Microsoft have announced that many of its features will be available as upgrades to XP anyway.