A few points:
-"Future-proofing" is ridiculously difficult and expensive. One could go out and drop ten thousand dollars on a top of the line PC; in three to four years it would be capable of little more than a mid-grade PC could do for a thousand. By far the most important part of trying to ensure you don't have to go out and buy a full new rig is to get base parts (Motherboard and case and especially) that have room for expansion (Extra RAM and PCIe slots and for the motherboard, ROOM to put any sort of funky shaped card inside of the case or any sort of cooling system~!) and to then upgrade on a yearly or bi-yearly basis. It's far more efficient and useful to get a new five hundred dollar video card every other year than to drop two grand on a state of the art card.
-It's also fairly hard to give a good build recommendation or discussion of part without a solid budget. There's a fairly large difference in what you can get built for a thousand dollars versus two or three thousand.
That being said, I can address a few specifics from your post:
If you are certain on Intel, then Core i7. It's Intel's flagship CPU. Adjust model as per price. Core i7 extreme if you can afford it.
A pair of GTX 680's in SLI could run ArmA2 two times over and still have enough clocktime left to do your taxes and balance your checkbook as well.
That's basically the equivelent of a modern physics proccessing card for fluid dynamics.
You'll be fine with 670's. More than fine.
If your motherboard supports it, the more RAM the better. RAM is cheap; While there are some people who might think there is such a thing as too much RAM, these people are what I like to call wrong.
Completely and totally wrong.
Unless you're a true audiophile, concerned about the difference between a 12 inch and a 14 inch sub, happen to own a pair of those fancy wood speakers, and get all your music on vinyl beacuse it's a better format (which it is, tbh), or a high-end professional sound mixer, the default sound pack on your motherboard will be just fine. Five ten years ago that wouldn't neccessarily have been true, but these days it really is, ESPECIALLY if you plan on using a nice set of headphones to game, in which case the drivers for said headphones will take care of everything a discrete sound card would.
Cooling is extremely important- Failure in the cooling system was the problem behind the first-gen Xbox RROD epidemic. Without cooling your parts will soon fry themselves into melty bits of plastic and metal.
As for watercooling..... I don't know man, I didn't do it.
Nobody seems to be quite sure if watercooling is actually neccesssary save for extreme overclocking. SInce I'm going to assume from your post you won't be overclocking the CPU itself to any terrible degree, it should be well within factory specs on heat and soforth. I
I mean, it's always NICE to KNOW one's computer isn't going to fry itself into a pile of scorched plastic and magic blue smoke, but is it really neccessary to do watercooling........... It's a toss-up, really. Sorry for being terribly unhelpful on this. But watercooling actually should be significantly quieter than all those fans if you do chose to do that.