You also have to count in voltage. generally 1.5 or lower is recommended for DDR3.
But besides that, CAS and mhz both don't have a big impact, unless its like 1600 and 2800 or something like that. A higher cas with a higher mhz balances out with a lower cas with a lower mhz. I mean, look at DDR4. The cas is very high, and so is the mhz. Some people have done tests with similar setups and funny enough for gaming, 1600 with CAS 9 was better than 3000 with like 13 CAS (though it was a very small amount)
Honestly, just make sure its at least 1600 and make sure the voltage is 1.5 or lower and you should be good. also, stick to trusted brands, usually the higher rated ones.
Also, you dont need more than 8gb if you are just gaming and not doing lots of video/art editing.