CAS2 PC133 memory modules are recommended for the somewhat newer systems from last year that run a 133 MHz memory bus speed since most 'normal' modules will not work properly at CAS2 with such a bus speed. Especially with VIA systems this can be very tricky.

CAS2 Latency, what is the hype about ? - A question that I hear a lot is what is the essential difference between CAS3 and CAS2. Simple answer, the effect is raw speed ! To explain CAS you really need to get an impression on how memory works, although I'm not a memory module expert I'll give it my best go to explain. 

You may have some PC133 memory already, or you might even be running some PC100 memory overclocked to 133MHz. Is this RAM different? Hmm yes, it is.

If we talk about SDRAM timing we mean two things. The first one that we encounter is the memory bus speed, MHz. In most cases this is equal to the  CPU's Front-Side-Bus (FSB) speed, except for newer VIA mainboards who can run an FSB of 100 and it's memory at 133 MHz.

The second item we need to set (and now we're getting closer to CAS2 memory) is of course the CAS latency in either a setting of  3 or 2. (CAS = Column Access Strobe) It handles to the part of the memory access cycle. To put it into geek words the CAS is a delay as  the SDRAM subsystem accesses the correct part of the memory. In other words, the lower the number (CAS2), the faster the memory access cycle will get. In return, the memory bandwidth will increase and thus we'll notice an increase in performance. The higher the CAS (3) the slower memory access will be. The majority of memory modules these days is CAS3 PC133. PC133-CAS3 is therefore guaranteed to work a FSB of 133 MHz but with a CAS Delay of 3. The reason why this module is so special is that it's guaranteed by the manufacturer to run at 133MHz at CAS2 and will probably go up to 143 MHz 2-2-2. This is truly good news for overclockers, tweakers and people who like to push the most out of their system since it'll bring us the option going back to CAS3 if we're pushing the RAM to much higher frequencies.