Yes, AMP can be run in 64-bit (for AWMS version 7.1+). AMP will be 64-bit only beginning with the 7.4 release.
If you are about to install AMP, the installation ISO on the download page is 64-bit, so new installs will be 64-bit only from this point on.
If you need to upgrade from 32-bit to 64-bit, please follow these migration instructions: https://arubanetworkskb.secure.force.com/pkb/articles/FAQ/Upgrading-Migrating-from-32-bit-to-64-bit
For versions before 7.1, you'll still be on the 32-bit OS until you go through the migration steps. For versions prior to 7.1, AMP could be installed on a server with 64-bit CPUs, but doesn't take advantage of their 64-bit capabilities. Should you run AMP with 32-bit on 64-bit hardware, remember to use the PAE kernel to recognize the additional memory.