Well, it's still a fairly new technology on the pc. And it's not true 64 bit it's mainly used for increasing ram now a days. It should take atleast two more years before we start seeing some good 64bit applications.
The thing about 64bit xp is that not all programs run on it, and some drivers are just imposible to find! Because xp is made to be a 32 bit OS, and they just changed it a little to add support for 64, it's not built for 64.