i would change parts to:
i7-6800k (replaces the i7-5820k, is slightly faster)
asus x99-a ii or better (the "ii" from asus are refreshed x99 motherboards for the new i7-6xxxx cpu's)
ddr4-2400 minimum, but recommended closer to 3000mhz (faster memory helps with gpu performance)
samsung 850 evo or pro ssd's. i would avoid software raid if possible. if you are doing it for speed, look at the samsung 950 pro. its a pcie m.2 nvme ssd and will be much faster than raid sata ssd. if you need 1tb worth of space, i would either motherboard raid the two sata ssd's or wait for the new samsung 1tb m.2 ssd's to come out.
the r9 390 is a fast card, but it may be slower in certain scenario's in premiere than an otherwise similar nvidia gtx card. if you don't have any software/plugins that need cuda, depending on your projects the r9 390 might be fast enough to keep. looking at nvidia cards, the gtx 1060 would be a good match to the 6 core cpu, even for 4k projects. it should be slightly faster than the r9 390 in raw power, but in premiere it should be even faster yet. the gtx 1070 gets into overkill territory for a 6 core intel cpu, but if you plan on using lots of gpu effects and gpu plugins it might help some.
liquid cooling will help for max overclocks, but will also require more expensive motherboards and power supplies to hit max overclock speeds and lower the risk of damaging hardware. if you are just going for medium to high overclock, a large air cooler can keep temps reasonable and avoid pump failures or liquid leaks. noctua d15s is popular, but requires a second fan for higher overclocks. you will also need a large enough case to fit large air coolers, and have enough case fans to keep the rest of the system and motherboard cool. if you don't want a large full size atx case, some mid size atx tower cases can do the job fine. the r9 390 may also be adding to higher temps inside the case, so good case air flow will be important to keep the gpu heat away from the cpu area of the motherboard. the gtx 1000 series cards will also help keep temps down as they use less power and using a single blower fan style will exhaust the hot air out of the back of the case.
Thank you for your advice RoninEdits! After having read threads about all the items on my "shopping list" for weeks now, allow me to be a bit nitpicky about your suggestions, especially considering the increase in price:
After having seen all the benchmarks for the Broadwell-E CPUs, I am rather sceptical if the additional price is worth the performance increase. Everyone is talking about a IPC performance increase of some 2-5% and yet you pay about 11-12% more for the current generation. Additionally, I am sure to go with a maximum overclock of 4.3-4,5 GHz on the 5820k if I win the silicon lottery. A 6800k runs a lot hotter with these clock speeds since a higher voltage is necessary.
Is it really worth it, paying the 10%+ more to get a slightly higher per cycle / MHz performance but worse OC potential?
Where is the benefit of using the refreshed boards, after most of the manufacturers have provided BIOS updates for using the Broadwell-E CPUs?
I just looked up some RAM benchmark results and agree with you that 2400 or 3000 is the way to go.
What is the benefit of using Samsung SSDs over the Crucial SSDs? I am most definitely looking for speed, but I already know, that I need at least 250GB for the OS + software and at least 700-900 GB for an active project. So a 950 pro (512GB) is not really an option besides using it as a boot drive which seems to be a waste of power. And a Toshiba 1TB M.2 is much to expensive (760€).
So my only option is to go with a 250-500GB + 2x500 R0 / 1TB configuration. I know about the risks of onboard raid 0 but a hardware raid card for two drives is a bit overkill in my opinion. Especially since I have some automated NAS drives for backup.
My R9 390 is a relic of my gaming days. I still play sometimes but the reason, I would switch cards at all is to improve the rendering performance. Yet don't think that the upcoming 1060 would be a huge improvement for me. After seeing the latest firestrike benchmark leaks, it seems like this card will be just 5-10% faster than a RX480 wich again has about the same performance as my 390. I don't want to go "full circle", so I will probably wait for the price of the 1070 to drop to a more reasonable ~400€ (considering the 970 started at 350...) and work with the 390 in the meantime. I Mostly use color correction, transitions, stabilizing and some blur as effects. Maybe I don't need to upgrade my GPU at all?
Deciding between the Liquid freezer and a Noctual NH-D15 has been a tough one for me over the last weeks. The Noctua is more expensive, cools about 5°C less, but is quieter and prone for problems. Is it really the better choice?
I am already sure, that I will use a full ATX sized case with space for fans in the front, the top and the rear.
And as a PSU I want to go with a 80+ Gold certified 700W PSU from Chieftec, BeQuiet, Corsair or another known brand. Since I already have a 580W BQ in my current system, I believe that 700W will be enough for my needs.
as you point out the the i7-6800k on average doesn't seem to be overclocking as high, say around 4.2-4.4ghz vs 4.4-4.6ghz on the 5820k. so a lower overclock with already low ipc gains would be a concern. intel seems to be setting ipc gains to 5-6% from what i have seen. the various non-premiere benchmarks i've seen for the new broadwell-e lineup suggests closer to 10% performance increase, so even with a lower overclock i would still expect at least 5% performance. if you are trying to stick to a budget and find a good deal on a 5820k combo with motherboard, it may be much better value. if you are going for max overclocks it will be hard to find sales or good value as it costs more for the quality parts that can do max overclocks.
the refreshed x99 motherboards tend to have a few new features for similar prices. some have reinforced pcie slots, some have just one for the primary gpu slot. some have better (full speed x4) m.2 slots and some have added u.2. the new refreshed asus motherboards also have different pcie lane splitting, x8/x8/x8 vs the old x16/x8/x4. features vary between different x99 motherboards, so it can take some shopping around to get specifically what you want. if you are going for max overclock it will require more expensive motherboards to handle the extra power.
there is alot of debate about amd vs nvidia with premiere, usually on this forum the popular opinion is that the amd cards are much slower. so if the gtx 1060 in every other gpu benchmark performs 15% faster than r9 390, gtx 970 and rx480, it may be 30% faster than the amd cards in premiere. since you are working in 1080p, any card around the r9 390 should be plenty for most projects and some 4k projects. so i would give it a chance and check the gpu usage (load) with GPU-Z to see if an upgrade is even needed.
samsung ssd's are known for best performance, reliability and endurance. just like when thinking cpu=intel or gpu=nvidia, when considering ssd it should default to samsung. two samsung 512gb ssd's in raid 0 will get you to 1tb and should be fast enough. i agree a hardware raid controller wouldn't be necessary, 2-3 drives in raid 0 is usually fine using the motherboard raid. any raid holding valuable data, including parity raid setups, should have an isolated backup copy. so hardware raid card vs software vs motherboard, doesn't make any difference for important data, they would all require another copy of the data somewhere else, like on your nas.
if you are going for max overclock, large air coolers won't be enough and you might want to look at the 280mm liquid coolers. you will also want to check how long the warranty lasts and what kind of damage it will cover if there is a leak. if you plan on keeping the system for several years or leaving it on rendering/exporting while you are away from the machine, you may not want to have a liquid cooler that could fail or leak at any time. air coolers only have fan failures to worry about and good ones tend to have lower noise (no pump noise, quiet fans). if an air cooler breaks its just a matter of replacing a fan vs replacing the whole liquid system.
for max overclock you are looking at 1,000-1,200 watt power supply, platinum or titanium. they will deliver a cleaner and more stable power current, and have better power protection systems to prevent damage to the motherboard and cpu. you can look up reviews for any models you are considering to help pick a quality unit. it may be hard to find a real good chieftec or be quiet unit. the corsair models get expensive and have cut quality on some of their units over the years, so its hit and miss on their various models. if you decide that gold is good enough, the rmi and rmx models are getting some good reviews. evga g2, p2 and t2 models have a very good oem and tend to have reasonable prices. seasonic is a really good oem that also offers their units for direct sales.