Well, technically the RAM sticks don't have to be identical to work in dual channel mode. However, it is preferred for stability reasons. And just like the example above, if you use two different ones, then the one with the slowest timings will force the other one to run slower too.
Also, many motherboards support 2, 3, or even 4 sticks running in dual channel. It doesn't have to be an even number of sticks as it would seem. Definitely install them in the configuration the mobo manual specifies.
~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
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