The upgrade will be available to anyone. You can download the public beta of XP 64. Check out MS site.
There will be only one version of XP x86/64. Intel will have to follow AMD standards or be without an OS if they introduce something different.
There is already a 64bit XP, but, it is for the Intel Itanium. That is a true 64bit processor, not based on x86.
Not to worry your system will not become obsolete when the giant releases their x86/64 chip.
The final release will be sometime this year. My bet is you'll see XP x86/64 be released shortly after Intel releases their chip which will be second quarter this year.
Longhorn is the OS for 2006.