DDR3 is triple channel Ram. However; P55/p67/H67 and Z68 chipsets do not allow triple channel mode and uses the DDR3 in dual channel mode. Not a big performance hit.
Any ddr3 above 1333 is Overclocked ram. This is just saying that the MB allows you to use upto DDR3 2600 ram.
Just for info: The sweet spot for SB CPUs is DDR3 1600 CL9. There is very little performance advantage above DDR3 1333, how ever the 1600 Ram is so close in price to 1333 that it becomes the defacto recommend speed. Above 1600 the price goes up and the performance increase is very little, not worth it.
DDR33 Explained: http://en.wikipedia.org/wiki/DDR3_SDRAM