My question is -- does the case have enough airflow for SLI?
This is a very tricky question, and will depend a lot into what games.... even the detail you run them at. My R4 isnt good enough to keep 2x GTX970 quiet under heavy load (unheaven engine), but in games like LOL/WOW is good enough.
Acer 4k Gsync
Compare the Acer XB280HK 4K G-Sync Monitor
with Acer B326HK 4K IPS Monitor
Items from existing buildHDD: Samsung 850 500gb, 840 250gb; can use 2tb WD green drive for data, but maybe skip?
Look into 850Evo, looks like like a top pick atm, specially for its price/performance.
Case: Antec P183
PSU: Antec CP850
GPU: Asus Strix 980 (one for now)
Since you are going for one gpu, iw ould stay with the P183/CP850 combo, once you buy the second i would check if the case is good enough or will you need to bump the speeds of the fans too much. Im buidling a twin GTX970 in a month or so on FT05, that should be as good as it gets for aircooling.
CPU: 5930k (overkill, I know)
I would go with the 5820K, unless you really going to need the 40 PCIe lanes, that most of us dont. And you can always upgrade toward a 5960X that would be a more considerable upgrade over the 5820K with more cores, once you need it.
Boot drive: Samsung XP941 m.2
Personally i dont see much gain into going m.2, yes it has higher bandwidth than a standard sata III ssd, but for me its extremly hard to pick between ssds in real world, my suggestion is skip it, wait for the sata express ssds in a year or two.
Yes, I know a 4970k is half the price of this, but I keep my builds for an average of 5-6 years. The "enthusiast" platform has served me well. I'm thinking DX12 will fly with this config, and when Skylake-E is released, I could pop in what should be a low-power/heat chip. At some point soon I'll need to go SLI or swap to Nvidia's next best offering -- 4k roughs up even the 980.
As long as you take advantage of what you are buying is fine, but personally i would prefer to ugprade more often than to try to future proof a build, a lot of good things are coming in the next few years that are not CPUs, like 20nm GPUs, PCIe 4.0, probably new formats of USB/TB, etc. From sandy bridge i have been upgrading just for the extras, the cpus bearly have improved, but there are support for much newer things, reason why i moved from 1366 platform.
Good luck with the build,