Dual Channel systems can be finicky about memory. You can have two similar chips and they might not work in dual channell mode. In theory, Dual channell memory must be similar to work and it usually will.
The reason that memory that is "specified dual channel" or whatever is that they are tested for compatability. For example, Corsair tests some of it's dual channell RAM (matched sticks sold as a set) memory in motherboards with NVidia nForce2 chipsets.
You pay more for the set because of the work that goes into matching them and guaranteeing that they will work in a dual channell system.
The Corsair XMS matched sets, while not the cheapest, usually work well.