When mixing speeds, all of the RAM will run at the slower speed.
667MHz is technically incorrect, though lots of places selling RAM call DDR2-667 RAM "667 MHz". The memory bus actually runs at 333 MHz and the RAM transfers data twice per clock, or 667 million data transfers per second (not "667 MHz").
Dual channel memory still runs at 333 MHz. In dual channel mode two sticks of RAM double the available bandwidth to the CPU. But that doesn't mean the CPU performance is doubled though. Dual channel mode is faster, but it isn't necessarily twice faster.
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