Depends on what time frame you consider the future. For instance, the gtx590 is one he'll of a powerful gpu, and it's not all that old considering. Yet sli is often a "future" upgrade possibility. Currently with SLI scaling in current games, power of a single gpu like a gtx970, SLI 590 just isn't worth it. For the price, SLI 660ti as compared to a gtx970 isn't worth it, and we are talking tech that's 3-5 years old.
The lga1150 is now a dead end. There are no possible upgrades. Of you decide to wait 3 years or so to get an i7-4790k, it'll probably cost close to what it costs today. A brand new i7-3770k is still over $300, laws of supply and demand.
The r9 280 is still a very popular card, yet it already has 1 successor in the r9 285, and now a 2nd in the r9 370/380, so what do you consider 'out of date'? If you equate that to the length of time the r9 280 has been out, your whole pc will be no different than a Sandy Bridge compared to Broadwell.
As many will tell you, there really is no such thing in electronics as' future-proof', all there is is get the best, fastest, strongest, most quality products you can afford, and hope they last a good long while, or at least until you are ready to upgrade again.
1. You can upgrade to an i7 or Xeon if available, at best.
2. Unknown. For all anyone knows, this latest batch of gpus maybe the last/best in pcie x16, and 2 years from now motherboards may see pcie x64 slots. Kinda like trying to upgrade from AGP to x16. Just won't.
3. Adding more ddr3 ram in the future may or may not work. Not all silicon is compatible, not all identical kits are compatible, secondary and tertiary timings will differ, that can and do often result in non working ram when mixed.
4. Unknown. A case is just a case, gpus and coolers may get bigger or smaller, or may dissappear entirely in favor of an msata or pci express style card. Who can say for sure?
5. 5 years ago, the biggest gpus required huge amounts of power, then along came Maxwell, and changed the world. 5 years from now, the biggest gpus may only require a 500w psu for sli, so, who knows. You may never need a bigger psu, wattage wise anyways. For all I know, ppl may be working on a new standard, making ATX obsolete, of which psus will be 1/2 the current size, with twice the capacity.