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I currently have 2x1GG 1333MHz DDR3 strips. I plan to purchase a further 2x4GG 1600Mhz strips. Will I be able to use both sets of RAM in my PC together? Will they both be limited to 1333Mhz?

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They should work, it will be limited to the lowest speed (1333 in this case), and you will not be able use dual channels with this setup... –  Kyle Oct 26 '11 at 18:59

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Mixing RAM sticks of different timings may or may not work. Memory has a lot of other timings besides the MHz rating and if the memory controller cannot find a set of timings that both sets of modules supports then your computer may not boot or may act very weirdly.

Otherwise it may be that the different set work perfectly well together, in which case they will work at the highest speed supported by the slowest module. In your case this will be 1333.

So buying 1600 RAM that you intend to use with 1333 will be a waste of money unless the 1600 is actually cheaper, which wouldn't surprise me these days.

If you truly are going to get 2x4Gb strips, then you're not going to notice much difference between 8Gb and 10Gb. I'd just chuck the 2x1Gb and let the 4Gb sticks run at full speed.

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Thanks. I Think I'll pull the 1Gb ones out. –  user940516 Oct 26 '11 at 19:16
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This will never be a problem because RAM always supports slower speeds or higher latencies. All of those specifications are limits. (You can have conflicts involving voltages, ECC/non-ECC, and the like, but not speed/latency.) –  David Schwartz Oct 26 '11 at 19:48
    
@DavidSchwartz I'm afraid you are wrong there, I've had memory which did not work well together as one pair supported CL=2.5 while the other supported CL=3, and none of their other timings quite matched up and as a result they didn't work together. –  Mokubai Oct 26 '11 at 20:03
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@Mokubai: You may have had some kind of problem, but unless your motherboard had a horribly broken BIOS or something, it can't be as you described. Anything your memory supports will be used only if it's compatible. Supported latencies are minimums -- it is required by the DDR3 specification. And every sensible BIOS will choose the lowest value that all modules support. (My bet is that you're repeating stories from ancient times as if they were still relevant. This question was about DDR3.) –  David Schwartz Oct 26 '11 at 20:26

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