RAM speed vs CAS timing

I currently have 4GB of DDR2 800MHz 4-4-4-12 RAM. I am looking to upgrade to 8GB of RAM (yes, I have a 64-bit OS) and can either add another 4GB of the same or upgrade to 1066MHz DDR2.

The difference is that the 1066MHz RAM has 5-5-5-15 timing.

Is there a big enough difference for me to buy 8GB of the 1066MHz 5-5-5-15, or is the difference so small that I'm better sticking with the 800MHz with the better CAS latency timing?
8 answers Last reply
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  1. You'd have to run benchmarks to detect the additional performance of 1066 MHz RAM.
  2. You are better off with the DDR2 800. You can slightly OC the DDR2 800 to reach the same performance as the 1066 at a cheaper price.
  3. Why do you need 8 gig of memory....?
  4. It's not always about need :)
  5. As my new best friend Bilbat learnt me this morning.

    At 1066, a ram bus cycle, rising edge to rising edge, is 1000/1066, or .936 nanoseconds; at 1333, it is 1000/1333, or .75 nanoseconds; (CAS)5 x .936 = 4.68 nanoseconds per memory transaction; (CAS)9 x .75 = 6.75 nanoseconds

    But in your case it would be 1000/800 = 1.25 ns * 4 = 5 ns per memory transaction.


    1000/1066 = 936 nanoseconds * 5 = 4.68 ns per memory transaction.

    So very very close on a theoretical latency basis.

    I'd say stick with the DDR2 4-4-4-12 at 800MHz.
  6. Thank you all for your responses, and thank God I comprehend the maths...

    I'll be sticking with the 800.

    And as far as 8GB is concerned - 64-bit OS, no pagefile.

    Oddly enough, this now means that this computer has as much ram as my first computer had hard drive...
  7. honolululu - Not disagreeing with you in end result.
    Math alittle off but comes out in the wash. @ Clock freq for DDR2-800 is 400 MHz
    which is 2.5 nansseconds. With DDR that means 1 transaction on leading edge and on transaction on laging edge. equivalant to SDR runing at 800 MHz. The 2 in DDR2 doubles that as the two sticks are effectively operating in Parallel - hense Fronside bus = 1600.

    Overall excellent comparrison. Don't forget that while the first digit(4-4-4-12) is most important the other numbers also effect performance
  8. Indeed they are all important - after starting this thread, I went on an information bender and found a number of reviews on my RAM (2x2GB kit of Crucial Ballistix DDR2 800MHz 4-4-4-12) some on Tom's.

    It's apparently one of the most overclockable RAM kits in its class:


    I made an awesome choice, and I wasn't even trying for it...

    (note that the review I linked is an older one, and states that the ram I have runs at 2.10v for the 800MHz 4-4-4-12 timing - the ram has since been upgraded and runs stable at that speed and timing at 2.00v)
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