Before I give you a little info to help you decide if you care about CAS, I have to ask this:
Where the hell did you see CAS6 memory? I don't think I've ever seen it for sale anywhere I've been!
Ok, now for the facts
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Tom's did an article comparing CAS to Mhz rating of RAM. Clearly if you buy CAS 4 ram, and set it to CAS 5 you can increase the RAM speed because you are adding a delay. Timings versus Mhz rating is a hot topic that is debated on many levels and with varying opinions. My personal thoughts are to buy good low CAS memory and run it at the designed speed you want. If you intend to use DDR2-1066, just buy DDR2-1066 or buy DDR-800 and overclock it adding in a CAS to make up the difference. On the other hand if you intend to overclock this machine to the limits, you want to buy the highest Mhz rating with the lowest CAS(effectively buying "higher quality" RAM) giving you a higher likelyhood of overclocking. Big picture the difference between CAS5 and 6 is about 2% performance if I remember correctly.
Read this article for some more info:
http://www.tomshardware.com/2006/03/31/tight_timings_vs_high_clock_frequencies/
Hope this helps.