Memory: trading off CAS latency versus speed

Question

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?

Answer 1

Crucial.com 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.

Answer 2

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

1. Memory timings and RAM?
   • 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.
   • “Faster” RAM at lower clock speeds?

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