You know how on 32-bit systems it can only use like 3gb of RAM? How much is that cap for 64-bit systems?
Theoretically, it's 2^64 = 18446744073709551616 bytes = 16777216 TB.
But physically, some hardware in your computer is designed only for memory sizes that will be common in next few years.
Most 64-bit microprocessors on the market today have an artificial limit on the amount of memory they can address, considerably lower than what might be expected from 64 bits. For example, the AMD64 architecture currently has a 52 bit limit on physical memory and only supports a 48-bit virtual address space.
Where 52 bits equals to 2^52 = 4503599627370496 bytes = 4096 TB and 48 bits = 256 TB.
192GB in higher-end Windows 7 systems, but at low as 2 in some cases :)