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Slots 3 and 7 should give you two 3x2GB triple-channel setups and the remaining 2GB in 4GB modules would run in single-channel. I think that would give you the maximum performance. Third 4GB module would be optimal though since then you could have two triple-channel setups without remainder RAM but you most likely won't notice any difference no matter how you install them (RAM bandwidth is not the bottleneck).
 
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