If you have a 1 x 512MB 533Mhz DDR2 RAM -your effective FSB is 533Mhz.

If you use 2 x 256MB 533 DDR2 RAM with dual channel, your effective FSB is 1066Mhz.

The total memory is the same for both cases, but just the difference with the FSB.

With dual channel, this allows you to achieve 1:1 ratio with your processor (assuming it's your CPU is Core2 Duo -1066Mhz)

Yes, dual channel uses 128bit. A single channel uses 64bit.
64x2 =128bit =dual channel.

If you run 3 x 256MB 533Mhz, you cannot run dual channel. It needs to be in a pair. ie 2 x 256MB 533Mhz or 4 x 256MB 533Mhz. Remember, dual channel gives an effective 128-bit wide data bus. So whether is 2x or 4x 256MB 533Mhz, you maximum FSB will still only be 1066MHZ.