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Last thanksgiving, I built a computer using this RAM. I have decided to upgrade to 8GB, so I just bought this.

As you can see, the 2 kits have different timings, voltages, and CLs. Is it possible use them together, either as is or by overclocking one somehow?

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To use them how, at all or as dual-channel?

You can use them together in general because the faster one will simply be slowed down to match the speed of the slower one.

As for dual-channel, you should be able to do it as long as you keep each matched pair in a separate channel (refer to your motherboard manual for slot-channel assignments).

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"As for dual-channel, you should be able to do it as long as you keep each matched pair in a separate channel" - this is backwards. For dual channel, a matched pair of modules must be installed in two different channels so that memory operations can be performed in parallel. If installed in the same channel as you wrote, then the mismatched modules in the other channel will force single-channel operations. Run a memory tester like Memtest86+ which will report the memory configuration plus a whole lot more. –  sawdust Oct 22 '12 at 21:31
    
Yes, MemTest and the BIOS POST should indicate whether it is working or not. As for the channels, I think it’s a matter of terminology. By “channel”, I mean pair of slots. For example, if the board has two black and two blue slots, (on most boards), you should put the modules from the same pair in the same color slots. I guess it makes sense that the paired slots would use different channels, but (1) what would be the correct terminology for the pair of slots then? and (2) it would imply that the different modules are being mixed and thus the faster ones slowed down to match the slower ones. –  Synetech Oct 22 '12 at 21:50
    
"By “channel”, I mean pair of slots" - That's not a reliable definition. What if there are six sockets? What about "tri-channel"? "Channel" is typically defined as a data path. Different motherboards color-code these channels in their own fashion. The mobos I've used (mostly Asus) have one color for each memory channel, so dual channel requires a matching module in each color. (1) Adjacent memory sockets typically correspond to the same memory bank. (2) Using mixed modules will always result in the slowest speed, but it's slow dual channel or slower single channel. –  sawdust Oct 23 '12 at 3:24
    
BTW wikipedia (en.wikipedia.org/wiki/Multi-channel_memory_architecture) seems to make a similar error in the photo caption, (mis)stating that modules should go in sockets of the same color, which corresponds to the same channel. I assert that matched modules should go in the same bank for dual channel, since that puts a module in each channel. You have to use two channels (in one bank) to achieve dual channel. The Wikipedia caption is incorrect. –  sawdust Oct 23 '12 at 3:31
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@ewok, The specs page only mentions 1.5V. I don’t see any mention of voltage in that list; I assume you looked up some of the models. (I suppose that there could be otherwise identical sticks with different voltages that use the same model number. Also, I’m fairly sure that the voltage indicated on product pages is the actual, operating voltage, not the maximum it can take.) Either way, I would recommend asking Gigabyte since there is enough inconsistency and confusion that they would (supposedly) be the only ones to know for sure. –  Synetech Oct 23 '12 at 17:41
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