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Two dual-channels with same speeds but different timings?

I have a PC with 2x2 GB DDR2-6400 800Mhz 6-6-6 sticks working in dual channel, and I need to get 2x2 GB more.
As far as I know that if I get sticks with the same speed as one another but a higher one than what I currently have, the speed will default back to 800. I was wondering however, would it be possible for me to get 2x2GB DDR2-6400 800Mhz 4-4-4 sticks to work in the second dual channel but actually achieve its expected latency (i.e., that the channel won't default to 6-6-6 timings because of the other channel)?
If it is possible, what do I have to set in the BIOS to get it done?
Otherwise I guess I'll just get a couple more of the same sticks I got now...

One more side-question - I might be getting this completely wrong, but according to what I've read, the "actual speed" of RAM is its latency, which can be be derived from its speed and timings. I've seen tables comparing different speeds+timings and their resulted latency, and it looks like while the clock rates have gone up over the years but the latencies pretty much stayed the same.
Basically what I'm asking is, is it true that memory "actual speed" hasn't really been improving much over the years? Would a DDR2-800 with 4-4-4 (10ns latency) indeed perform better than a DDR3-2133 11-11-11 (10.3125ns latency)? Or am I just getting this wrong?

Thank you very much.
4 answers Last reply Best Answer
More about dual channels speeds timings
  1. You got the basics right (thanks to wikipedia). Performance improvement are mostly based on dual channel and spreading the writes to different "pages". Base line: your ram will run at the speed of the slowest stick and that doesn't matter. You would not see any difference on the desktop. Did you make sure, your mobo supports 8Gb?
  2. Best answer
    No you can't set separate timings for individual channels. Timings are set for all modules at once. So if you get 800MHz 4-4-4 sticks you should choose 6-6-6 in BIOS to match the other ones. You can try to OC the older modules to 4-4-4 but this is unlikely to be stable.

    Time to access data stored in memory hasn't changed much, it's true. But DDR3 architecture offers some technical advantages over DDR2, and it will offer higher bandwidth. Actual speed of memory is much more complex to calculate than its initial data access time.
  3. As the previous posters have already stated, you have to match the timings to the lowest denominator, so you would have to set your ram timings to the lowest performing ones, which would be the 6-6-6 you currently have.

    Also, do make note of the voltage needed to run the memory, as this can vary too, your current would likely require 1.8 and the 4-4-4 most likely 2.0 - 2.2.
  4. Best answer selected by Cresfy.
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