What is the maximum amount of ram a 64bit machine can theoretically address?

Effectively, yes - processes could, in theory, address 2^64 bytes of memory. But as you pointed out, there are ways around this limit.

"Would be also good to note that the operating system has its own limitation about memory in a 64-bit architecture.

For example, see what wikipedia sais about Windows Vista 64:

All 64-bit versions of Microsoft
operating systems currently impose a
16 TB limit on address space.
Processes created on the 64-bit
editions of Windows Vista can have 8
TB in virtual memory for user
processes and 8 TB for kernel
processes to create a virtual memory
of 16 TB.[29] In terms of physical
memory Windows Vista 64-Bit Basic
supports up to 8 GB of RAM, Windows
Vista 64-Bit Home Premium supports up
to 16 GB of RAM, and Windows Vista
64-Bit Business/Enterprise/Ultimate
supports up to 128 GB of RAM.[8]"

The biggest advantage to 64 bits is not the RAM it can address, but everything else. You can define an address for every byte on a disk, for example, and increasing disk capacities will not invalidate this for decades.

Most of today's current processors have some sort of artificial limit on their address size. For example, the AMD64 architecture has a 52-bit limit on physical memory and currently only supports a 48-bit virtual address space. (Via Wikipedia). However yes, physically ~16.4 million terabytes is possible.

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