Windows 8 supports up to 192GB
http://social.technet.microsoft.com/Forums/windows/en-US/05c7197e-e567-44d2-be9e-b8018ec4438a/what-is-the-maximum-amount-of-installed-ram-that-windows-7-x64-can-use
I was recently in contact with a Cubase User. He has 128GB installed in his HP Z- Workstation.
Everything was working like a charm.
Lets see if Apple will support more RAM in 10.9. Or the nearer future.
Best regards,
Marcus