Did you read the Wiki page?http://en.wikipedia.org/wiki/CAS_latency
Cas 2 and cas 3 are orignal DDR timings. The fastest DDR3 1600 I have seen is cas 6 and that's rare. I have cas 7 DDR3 1600.
It really depends on what you are building as to the memory that's best suited for the build. 1.5v DDR3 1600 cas 9 is overall the best "bang for your buck".