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Originally Posted by DaWei
The 64-bit thangy is the width of the data bus. The wider the bus, the more data you can transfer with a clock edge. That makes for a performance increase. While machines may have an address bus the same width, it ain't necessarily so. The width of the address bus, how many of them are brought outside the device, and how many are wired up will determine the general memory space. Some systems will set aside part of the space available for memory-mapped I/O, also.
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I thought terms like 16-bit, 32-bit, etc referred to the native word size of the CPU, not the bus widths (though on many, the latter are at least equal to the former). For example, the 80486 and later x86 chips, with the embedded FPU, could often do atomic writes larger than 32 bits (ie, a wider data bus), though they were 32-bit processors. Likewise, certain chips like the Intel 80386SX or Motorola 68K had a data bus of only 16 bits (and sometimes a more limited address bus as well), though the chips were otherwise 32-bit.