The main differences between the 32-bit versions of Windows Vista and the 64-bit versions of Windows Vista relate to memory accessibility, memory management, and enhanced security features. The security features that are available in the 64-bit versions of Windows Vista include the following:

1.)Kernel Patch Protection
2.)Support for hardware-backed Data Execution Protection (DEP)
3.)Mandatory driver signing
4.)Removal of support for 32-bit drivers
5.)Removal of the 16-bit subsystem

One of the greatest advantages of using a 64-bit version of Windows Vista is the ability to access physical memory (RAM) that is above the 4-gigabyte (GB) range. This physical memory is not addressable by 32-bit versions of Windows Vista.

Depending on the version of Windows Vista that is installed, a 64-bit version of Windows Vista supports from 1 GB of RAM to more than 128 GB of RAM. The ability to address more physical memory lets Windows Vista minimize the time that is required to swap processes in and out of physical memory. Therefore, Windows Vista can manage processes more efficiently. This memory management feature helps improve the overall performance of Windows Vista.

Source:
http://support.microsoft.com/kb/946765