How much memory can you put into one giant x64 machine and actually use it?

The answer is: 2 Terabyte.

"You can see physical memory support licensing differentiation across the server

SKUs for all versions of Windows. For example, the 32-bit version of Windows Server

2008 Standard supports only 4GB, while the 32-bit Windows Server 2008 Datacenter supports

64GB. Likewise, the 64-bit Windows Server 2008 Standard supports 32GB and the 64-bit

Windows Server 2008 Datacenter can handle a whopping 2TB. There aren't many 2TB systems

out there, but the Windows Server Performance Team knows of a couple, including one

they had in their lab at one point. Here's a screenshot of Task Manager running on

that system:"