That's 16,777,216 GB, in case you're wondering. @_@
Actually Adam Novagen, the 64 bit processor doesn't have a 64bit addressing mode, but at 40 bit addressing mode.
http://www.theinquirer.net/en/inquirer/news/2004/07/13/intel-nocona-bug--points-way-to-amd64-origin
This translates into a theoretical 1 terabyte maximum ram on AMD64 architecture, but on some intel EM64T processor it translates into 64 gigs addressability (36bit address space).
http://en.wikipedia.org/wiki/64-bit
the AMD Athlon X2 has a 40-bit address bus and recongizes only 48 bits of the 64-bit virtual address[1]. The newer Barcelona X4 supports a 48-bit of physical address and 48 bits of the 64-bit virtual address.