Keep in mind, 256GB of RAM is not an ASL limitation - its the limit of ALL current x86_64 bit architectures no matter what OS, CPU, MB, you use - Windows, Linux, MacOS, etc. But even this is just a theoretical limit as a motherboards must be able to support that big of a memory footprint, and operating systems place much lower limits on the address space. See http://en.wikipedia.org/wiki/X86-64
For example, in RHEL 5 the maximum for the x86_64 arch is 256GB supported:http://www.redhat.com/rhel/compare/
Another example, with Windows 7, the maximum is only 192 GB:http://www.zdnet.com/blog/hardware/max- ... ows-7/4254
As the hardware is capable of handling larger memory spaces on 64 bit platforms, so will ASL. So on 64 bit you have no limitations with memory with ASL, you're only limited by your hardware and OS.