While it is physically possible to install the 64-bit version of Windows XP, you face a problem when it comes to drivers. Boot Camp only supplies drivers for the 32-bit versions of Windows XP and Vista, which is why Apple states that you should only install those versions. The only exception are the latest Mac Pros, where 64-bit drivers were recently made available.
Most of the parts in your Macbook Pro would probably be recognized by Windows XP x64, but things like iSight, Bluetooth, and the trackpad won't. So while Windows XP x64 would technically provide a boost in performance, you'll face a lot of issues when trying to install it without adequate drivers.
Perhaps a future Boot Camp update (possibly in the upcoming 10.5.2) will add the drivers and support for x64 and the 64-bit version of Vista.