I built my current rig in 2008 with future proofing in mind, and although a bit dated, it serves me well almost 10 years later. The only hardware updates that I have done so far is the processor, going from a 2-core cpu to the quad-core Q9550, and upgrading the video card to the current EVGA GeForce GTX-760. I also switched from WinXP to Windows7 64-bit Professional...which made a huge difference in performance. I also got a huge gain in performance by simply overclocking the cpu 2.83ghz to 3.2ghz.
*Antec 900 case
*Antec 850 watt PS
*EVGA 132-CK-NF78 Mobo
*8 gigs Corsair RAM
*Intel Q9550 Quad-core overclocked to 3.2ghz - Lots of air cooling for this.
*Several HDD and Optical drives.
I'm currently limited by CPU thru-put, so upgrading the vid-card beyond what I have now wouldn't net me any gains. I can play any game in my gaming library without issues though, and the only issue I currently have is watching 4K content, as the video occasionally stutters a bit if I have other background programs running. I'll probably upgrade towards the end of the year after the progression of prices drops a bit more. One thing is for certain, it is getting harder and harder to plan a "future-proof" build with the rate that technology is advancing. I doubt I will ever get close to 10 years out of another build.