Generally speaking, I like to get faster, smaller amounts of ram. For example. Some people might build a modern rig on a motherboard with 4 slots of DDR3 that can hold a max or 32G. And they'll put 32G of CAS 11 1333 memory in there. I see that a lot actually. Sure it works just peachy. A lot of modern boards though can handle a lot more, without even overclocking. I.E. I would rather stick with 16G at 1866, CAS 8 (for gaming).
You have three items; Size, CAS and Speed.
CAS is the latency of the memory, or how long it takes to respond to commands. So for gaming memory, lower CAS is better. But SPEED is also critical, higher speed is simply that. So there's a trade off, A CAS 9 at 1600 is still better then a CAS 8 at 1333... So it can get confusing.
To make it simple, do this. Look up your motherboard. See what it's memory speed is (if you don't want to overclock it, that's perfectly fine, it will have a posted max limit. A lot of modern ones do like 1866 just peachy today without overclocking).
Then go shopping for that speed of memory in the amount you want. If you have 4 slots and you want to fill it up, there ya go. OR if you don't want to fill it up but you want faster, you can use all four slots at half of their capable size for a lower CAS value, that's what I usually do. If you were making a video editing rig, CAS isn't quite as important as raw size and speed oddly enough, it all comes down to intent of the box.
As for brands, so many good ones out there. My go-to brands right now are G.SKILL and Mushkin just because I use so much of em. Oddly enough I used to use a lot of Corsair but, well, I'll just say I've lost my taste for them lately and leave it at that. Still use their power supplies though (which really, for the most part are Seasonic's anyhow).
But there ya go, everything you need to know to get the memory you want for your rig. Hope it helps.