kevindelaney,
Latency is measured in terms of clock cycles. For example, a CL=2 part requires two clock cycles to respond, while a CL=3 part requires three clock cycles. Thus, CL=2 parts complete the initial data access a faster (only 2/3 or the time) than CL=3 parts.
CAS Latency. This sets the CAS latency timing of the DRAM system memory access cycle when SDRAM system memory is installed.
If you would like to know a little more about the difference in performance you can read this review by Anandtech:
Mushkin & Adata: 2 for the Fast-Timings Lane
When you have two sticks RAM in your computer, the computer will default to the slowest stick's speed and timing unless you go in and set the speeds and timings manually. In your case the BIOS will default the speed of your RAM to CAS 2.5 PC-2100.
If you run WinXP you really need 512Mb of RAM. Running 128Mb just chokes it. You will see a much bigger boost in performance by having 512Mb of RAM than you lose going from CAS 2 to CAS 2.5.
Take Care,
Richard