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If your going to go for very good RAM like Micron, you might as well do Cas2, unless the only reason you're even considering Micron is because you can get it for what you usually have to pay for PNY, Kingston, etc. Is Micron Cas3 better than, say, PNY Cas2 for example. Maybe. I would still pay the extra few buck's for the Micron Cas2.
The link to Micron direct for the 15% is also getting the free shipping Micron itself is offering. I would imagine the only improvement one could get is another 5% eBates assuming they let you get the 15% off instead of Micron's usual 10% off for web sales prices. Less than $5 for one stick of 128mb PC133 cas2....not worth it to let another company have my info. Now if you buy a lot of RAM or already have eBates account, why not try.
Bluesky: There is a difference in running at cas2 over 3. It may not be noticeable doing just web surfing. You get to gaming, graphics, and more CPU intensive operations, then you will notice. More important is it if you intend to overclock. Example: PC100 cas2 may need to be used at cas3 to overclock for 133 for most RAM, but if you already have cas3 PC100, it won't even boot up for 133. Just an opinion gathered from reading others takes on subject as I have yet to play with overclocking myself.