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I am trying to upgrade my server's memory for a Dell precision 450 machine. The memory I have now (1GB in two 512 KB sticks) is DDR ECC PC2100 / CAS 2 / 266 MHz (Kingston KVR266x72c2). I want to get two sticks of 1GB each to bring me to 3GB total. It appears buying faster memory will be cheaper due to larger production.

Options I see at newegg are:

KVR266X72C25 which is CAS 2.5 and speed 266 MHz
KVR333X72C25 which is CAS 2.5 and speed 333 MHz
KVR400X72C3A which is CAS 3   and speed 400 MHz

They are all pretty much the same price ~$35 so am just looking for the one which will run best on my system.

My current memory is CAS 2, but there is none sold as CAS 2 in 266 MHz, so I wonder if CAS3 at 400 MHz is actually better latency since it is 400 MHz so 3 cycles is less than 2 cycles at 266 MHz, or is CAS 2.5 at 333 MHz better?

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Have ordered the: KVR400X72C3A which is CAS 3 and speed 400 MHz – WilliamKF Nov 8 '09 at 1:23
up vote 1 down vote accepted recommends this:

DDR PC3200 • CL=3 • Unbuffered • ECC • DDR400

i suggest you remove the old PC2100, and go with 400 MHz DDR RAM only rather than mixing them.

edit: download and run SIW, under Hardware click on Memory, depending on the mainboard it might show you the supported frequencies.

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Why, then I'd only have 2GB instead of 3GB? I think the mother board only takes advantage of 266 MHz, so faster speed memory won't be utilized at its fully rated speed. – WilliamKF Nov 7 '09 at 21:50
you think or you know? it will definitely run at 266 MHz if you mix them, but there is that possibility that you'll get 400 MHz if you're using 400 MHz RAM only (why else would recommend it then?) and if that is the case, you might as well add another GB of 400 MHz RAM later. if it doesn't work at 400 MHz, well, then you can still mix them and stick to 3 GB @ 266 MHz – Molly7244 Nov 7 '09 at 22:02
Don't know, but suspect it, given that the original memory in the machine when it was new was 266. They might recommend the 400 mhz because it is less expensive. – WilliamKF Nov 7 '09 at 22:09
i have a newer Dell Dimension here right now that came originally with PC4200 (533 MHz). when i replaced it with PC5300 (checked with crucial) it automatically switched to 667 MHz. Dell BIOS usually don't have any options when it comes to overclocking, so it either works or it doesn't, it's pretty much trial and error. as i said, worst case would be running your 400 MHz DDR ram @ 266 MHz. and off the record: i always collect old memory and those older Dell machines are quite grateful, they take pretty much anything that fits into the socket without complaints. – Molly – Molly7244 Nov 7 '09 at 23:53
It is not uncommon for manufacturers to put slower RAM in machines that support faster, if the machine is made at a time when the faster RAM is noticeably more expensive. I would see if you can find the motherboard's make/model/part numbers (they are often etched onto the board itself, and may be presented to you during the power-on-self-test or in the BIOS configuration screens) - with that info you can look for documentation that will state which RAM speeds the board will support. – David Spillett Nov 8 '09 at 0:02

Suggest you look at a few related questions and answers (on the right here).
This is one short list,

  1. Memory timings and RAM?
  2. Is it safe using RAMs with different Frequencies
    And I guess, you are not trying to do this.
    You are working with different slot pairs for your new and old memory.
  3. “Faster” RAM at lower clock speeds?

Further reading at TomsHardware Memory FAQ
Look for the Latency and Bandwidth section halfway through the page.

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