Dual Channel RAM
Depending on the application, Dual Channel ram can be anywhere from 1% to 10% faster than single channel.
Ram speeds are determined by your northbridge chipset. The speed on the ram only indicates what it is 'rated' for.
Putting DDR 1600 in a computer that maxes out at 1333 will give you 0 benefit. The advantage to faster ram comes if you are over clocking your bus.
That said, you should always buy RAM that is rated for at least what the northbridge will support. My Mac book pro clocked down from 1600 to 1333 when I upgraded one of my RAM sticks.
The differences in RAM is so slight, that you will see much bigger return on investment if you splurge and get an SSD.