Cas latency is the amount of time it takes for the memory module and memory controller ot communicate, that is a very general description.
you should not really pay attention to those things unless you are a power-user or a benchmarking, in gaming your cpu/gpu are so strong they will give you what you need, the difference in latency would mean maybe 3 fps difference, you are not going to notice that if you are already getting 60fps from your cpu/gpu.
as a gamer you want your money to go toward the psu, gpu and cpu, those are the ones that are going to make the biggest difference in your gaming performance.
to answer some of your questions:
haf's filters, even cases with filters will get dust thru, if you literary clean for 2 minutes a week, dust won't matter, also put your pc high from the ground if you can on a desk, dust won't hit you no where near as hard if you are higher off the ground, and I have pets so trust me I know both dust and hair very well. Now having said all that, they make custom filters for the haf or you can just get a fan filter for a 230mm fan, or there are many old school tricks.
the memory you chose cust an arm and a leg cause its made for overcloking, now you don't want to oc your ram if you don't know what you are doing, it can cause you headache, and again its made for serious overclockers, you only need a good ram nothing fancy for overclocking.
chech this page its gone some choices for you, the Ballistix is a good ram and what you need at a fraction of the cost .
psu = power supply. HX is model of corsair's psu. go with corsair 750w HX model, the 850 HX is suppose to be a great one also.
as for your gpu, what is the native resolution of the monitor you gonna be gaming at? we need to know this so we can figure out what gpu you need, you don't want to waste your money on power you will never see and you don't want to endup short either, knowing what resolution you will be gaming at, will help us decide how much gpu power you need.
if you need to know how to post links let me know, I can show you that also.