The GTX680 will be a little above the GTX670, it's a very similar card (same chip and similar other specs).
The GTX690 should have no problems maxing it out, but then it's hardly a mainstream solution.
@ RAM, I have no idea. I'm sure 4Gb would be perfectly sufficient, like I say most people these days go for 8Gb as it's cheap.
Depending on the game, the CPU isn't especially well used. Most games use at best two cores, so any advantage from having 6 or 8 cores (or threads) is basically wasted. The intel core i7/i5/i3 line use similar tech, so in games there is little to choose between them. The high end k-series processors (like the 2500K you mentioned, and the slightly more modern 3570K) will pull ahead of their brethren after they are overclocked, but at stock speeds (in games) they are near identical to anything from the same family. If you are asking what the cheapest processor that can max out this game is, then it's likely to be a lowly i3 2100/3220 or an FX6300 matched up with a monster graphics card.
http://media.bestofmicro.com/9/G/364516/original/CPU-scaling.png
http://www.tomshardware.co.uk/far-cry-3-performance-benchmark,review-32584.html