Memory: trading off CAS latency versus speed

Solution 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.

Solution 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.