sli means to run 2 or 3 gfx cards linked together. it enables the use of 3 or more monitors instead of 2 like this 1 can do, but it doesn't mean you get 2x or 3x the performance of the 1 gpu, i'm not sure exactly but i know 2 cards isn't 2 times the performance.
overclocking i making your cpu ang gpu, run at faster speeds than normal, like with that cooler you can take this 3.3ghz cpu and turn it into a 4.5ghz cpu, and on the stock cooler you can prob get it close to a 3.6-7ghz cpu.
for that case above, it's one i linked, idk why you're saying it seems legit, cuz it is, but i'd still say use the haf x, just a beast, or a corsair 600 white edition, black is good too but i love the way the white looks, also the lian li's are supposedly the highest quality cases out there, and i do have to say they look nice and are reviewed amaizingly, but i'm personally not too fand of the looks, but that's just me.