RAM; windows and a few running programs can easily chew up 6-7GB. Don't ask me how. My "idle" (i.e. mail client, skype, browser running but only one tab) ram usage is 4.9GB.
As you install crap that runs in the background, it'll always grow a bit.
16GB is not waste, believe me. It ensures that windows won't force stuff on that stupid ramdisk/cachefile (what was it called?).
Playing on SLI; I've had SLI before, just not 970 - I was using 2x670. And something else before that. But SLI for long generations now, I've even done it with Ati cards
The first card always runs a bit hotter, and if your mobo layout puts the two cards side-by-side with no gap, that can further increase the heat. Some cards can overheat that way. Some gets hot but can tolerate it.
The current mobo I'm using has a gap between just for this reason.
The current case I'm using (Thermaltake Fortress ft02) was also selected for best airflow & positive air-pressure (plus to reduce dust).
I'm also sporting a h80i watercooler on the cpu; The main advantage is that it takes the heat out of the case, so other components (gfx cards & mobo chipset) can breathe easier. Otherwise the water does not cool too much better, compared to a good air cooler (it's a common misconception - and true if compared to the shitty default coolers).
As for noise: a big fanless cooler or one with a nice big low-rev fan inside the case will be actually more quiet to you than the WC's fan right outside the case.
But for a good air-cooler, you'll have to fork out nearly the same amount of money! And it's a big hunk of sharp metal, that can even damage a motherboard in extreme cases, being too heavy; and it blocks access to other parts, too, it can restrict your choices of ram (space constraints)...etc.
However, if you never plan to overclock, you can fairly safely stick to the stock one. As long as the case have a decent airflow it'll be ok, unless your cpu dissipates an unusual amount of heat. (there were a few models that were good for cooking eggs...)
Now as for the cost benefit; if you're buying a low-ish cpu, then it's sometimes indeed cheaper to upgrade that instead of buying fancy coolers and o/c. From the mid-range up, it's borderline, high-end will definitely be more cost effective to overclock with goood cooling.
Plus the wc has the added benefit of keeping other components cooler, as I said.
But again, with one good (as in not running hot) gfx card, and a non-oc cpu you need not worry about wc. Hence my recommendation of a 970
It's not too power hungry, runs cool and has power to serve you well for years. Sound investment IMO
Edit: fark, this got long. Sorry, hehe.
