Title sums it all up. My Q6600 has lasted god-knows-how many years, and it's still kicking. Of course, it's not the beast it once was, but it still plays many games. Note the word many, as the Q6600 has been showing its age for quite a while *cough*BF4*cough*. Either way, my Q6600 idles at about 65-75% cpu usage in FC4, and when things get serious, it goes up to 90%, but I've never seen 100% come out of it. Funny, seeing as how the minimum requirement for FC4 is an i5-750.
Plus, the first ever i7, the i7 920, is STILL a beast despite being released in 2008. If overclocked to 3.5GHz+, it can handle every card today. I'm pretty confident that it can handle a Titan X.
Needless to say, for a futureproof rig, dont get an i5 and SLI 980 Ti's. Get a high-end cpu (FX-9xxx or 6th gen i7) and atleast a midrange gpu. I guarantee you that it would last a very, very long time.