What is CAS? What is the performance difference between CL2 and CL3?
"CAS" is short for "Column Address Strobe". A DRAM memory can be thought of as a matrix, kind of like a spreadsheet with memory cells instead of numbers and formulas. Like the spreadsheet, each cell has a row address and a column address (like "AA57" or "R23C34" in the spreadsheet). There is also a RAS signal, which is stand for "Row Address Strobe".
To understand how CAS works it is best to walk through a simplified version of how the memory is actually read. First, the chip set accesses the ROW of the memory matrix by putting an address on the memory's address pins and activating the RAS signal. It then waits a few clock cycles (known as RAS-to-CAS Delay). Then, the column address is put on the address pins, and the CAS signal is activated, to access the correct COLUMN of the memory matrix. Then, it waits a few clock cycles (this is known as CAS latency) and then the data appears on the pins of the RAM.
So what's the bottom line... is one better than another?
CL2 parts process data a little quicker than CL3 parts in that you have to wait one less clock cycle for the initial data. However, after the first piece of data is processed, the rest of the data is processed at equal speeds. Latency only affects the initial burst of data. Once data starts flowing, there is no effect. Additionally, most systems will accept either latency part. Bear in mind, a clock cycle for a PC133 module is typically 7.5 nanoseconds, so the difference between CAS latency 2 and CAS latency 3 for PC133 parts is 8 billionths of a second. If you can notice that small a difference we highly suggest you apply for a job at NASA! :)