5820K with X99 for now and overclock to 4.3Ghz+. Then in about 4-5 years, find a used top-of-the-line Broadwell-E drop in upgrade into the same system.
Alternatively, I would do as others suggested - upgrade more often. i7 6700K and in 3-4 years another new system and another in 3-4 years. But if you prefer to not upgrade for 7-8 years at a time, then go X99 as it has the most long-term potential as far as 6-8 core CPU upgrade path, and later upgrade drops for Xeons much like X58 users are enjoying their $100 6-core Xeons today.
Both suggestions are dead on. The part of the build that has always kept its value best is the leftover budget. If the computer market is still recognizable as such, it will keep its value best.
Next best is a motherboard with a socket that will support cheap as chips workstation/server Xeon pulls. Xeon prices fall
hard, and they fall from high enough that there's some serious performance to be had there.
Don't worry about video cards, in five or six years your PCIe slots will nearly definitely be more valuable empty than full with whatever you put in them. I'd gun for price/performance, and I'd wait to see what's coming soon.
RAM is a tricky subject. I tend to feel the non-high end kits hold value well enough it's worth not skimping lest you have to replace sticks (especially if you're playing with four channel sets), but I'd defer to actual analysis.
The PSU will last. If you've got a good one, keep it. If you don't, buy a good one to last. Same for case. Your PSU is old enough that I think a new one might be worth a look, the newest crop is very good. The case is dated, but that's at least partly fashion.
Storage is interesting. I'd get a mix of SSD and platter to suit what you do, and check in on the market every once in a while to see if there's a major capability jump available. A good PCIe M.2 slot will help here, as well as NVMe boot capability, but that might be a wait and there's always
something to wait for.