Dual Channel doesn't double the speed of the RAM, it just doubles the data throughput, by utilizing two 64-bit data pathways instead of one, equaling 128 bit bandwidth between the RAM and processor.
So, if your motherboard supports RAM of speeds up to 800Mhz (PC6400), and you slot two PC6400 RAM modules into your motherboard, all your RAM will operate at this clock speed.
As for the second question, if you place 4 of the same specification RAM modules in the DIMM slots, then dual channel should work, since you've got pairs of RAM modules with the same specification (capacity, clock speed, chip configuration, latency etc...)...
...However, if for example, you place 3 x of the same spec. RAM modules in the DIMM slots, and 1 odd RAM module in the motherboard which is different, (capacity, clock speed, chip configuration, latency etc...) then the memory controller will operate in single channel mode.
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