The X11 protocol that underlies the X Window System is largely a 32-bit
aligned protocol. Because it is a standard, the X11 protocol
is fixed and does not change when machines are upgraded to 64-bit
operating systems. There is
not any justifiable need for a 64-bit version of the X11 protocol.
Window system operations work perfectly well manipulating 32-bit
values.
It is worth considering if the X server needs to run as a 64-bit process on a 64-bit system. On systems like Digital's Alpha where 64-bit is the only option, the X server necessarily runs as a 64-bit process. Excepting the case where only 64-bit is supported, the X server does not need to run as a 64-bit process. On systems that also support 32-bit processes like SGI's IRIX 6.0.1, the X server does operate as a 32-bit process. Indeed, by running as a 32-bit process, the memory footprint of the X server is smaller than if it ran as a 64-bit process.