64 Bit OS and RAM

The theoretical 64-bit address space is 2^64 or 16 exabytes (18,446,744,073,709,551,616 bytes)

Microsoft limits the maximum physical memory to 512 GB for Windows 8 Pro and Enterprise. (reference)

This is mostly for commercial reasons although I have heard that Microsoft will only authorise up to the memory size for which there exists equipment they can test.

As far as the per-process user mode virtual address space, this was limited to 8192 GB prior to WIndows 8.1. Windows 8.1 has increased this to 128TB. (reference) (reference)

The way the virtual address space is laid out is a result of the way that current processors only implement 256TB of virtual memory.

While it could address much more RAM in theory, Microsoft limits the possible RAM for Windows 8 to 512GB in the Pro and Enterprise editions, and even less in the others.

Windows Server 2012 is required to get to several terabytes of memory, which is still below the limit of appropriate hardware.

It's probably safe to assume a non-technical reason, i.e. they want you to buy a real server OS for server (or related) tasks.

Note that CPUs have their own limit for addressable memory as well. Intel e.g. specifies a limit of 750GB for current Xeon E5-2600 (designed for dual socket boards) CPUs. To get to >1TB, you need the (more expensive) ones designed for 4+ CPU boards.

Well, your sources are right even if they aren't Microsoft. The NT kernel will map this to different address regions, but even current processors only implement 48/52 Bits, which limits the amount to something below the theoretical maximum of the 64 Bit architecture.

Also, don't confuse the idea of virtual memory a program can use with the physical RAM the operating system can access through the CPU. You might want to read up on concepts such as Nonpaged and Paged Pool in Russinovich's "Windows Internals" as well as about how Paged Pool actually gets used to represent the process space for usermode processes.