Speed and width - two properties affect amount of information transferred.

  Speed - number of clock cycles per second.
    Kilohertz KHz    1 * 10^3 cycles/sec
      external devices, modem.

    Megahertz (mu)Hz 1 * 10^6 cycles/sec  
      early PC CPUs.

    Gigahertz GHz    1 * 10^9 cycles/sec 
       current CPUs.


  Width - number of address and data lines.
    Address - amount of primary data directly addressable.
      20 bit = 2^20 1 Megabyte Original IBM PC.
      36 bit = 2^36 64 Gigabytes - current CPUs.
      44 bit = 2^44 16 Terabyte Itanium (servers)

    * Because modern memory data buses are 8 bytes wide, the lowest 3 bits 
      of an address don't need to be represented on the address bus.

      This means a CPU with a 32 bit address pin-out can actually address
        2^35 or 32 GB of memory with right design.

    Data - amount (size) of data transferred at one read/write access. 
      2^8 - 8 bit bytes 2^8 0-255 (IBM PC, Apple, Commodore)
      2^16 - 16 bit word 2^16 0-65535 - 8086 CPU 
      2^64 - 64 bit word (8 byte) (current Pentium CPUs)